They will give you recipes and, very occasionally, small gifts via mail.

Being a game where you do the things that you want to do though, if making friends isn't what you want to do, don't do it.

However, arguably the most important thing that making friends does is it gives…

The game's story

Being a farming RPG, arguably just making friends is part of the benefit. There is a story (more story than “plot” since it's not linear) to Stardew Valley, and it's revealed by getting to know the NPCs.

Not making friends is like running around in Skyrim without ever doing the main quest: totally doable and it can be lots of fun and entirely fulfilling for a given player, but from an objective measure it results in missing a significant percentage of the game's content. (No backhanded criticism is implied here! This is how I personally play Elder Scrolls games.)

But again, this is a matter of taste. Getting to know the NPCs can be fulfilling in the same way that playing To The Moon is. But for those who like the Tycoon-game aspects of Stardew Valley more than the To The Moon aspects, the game is clever in allowing the player to find and focus on the things they personally find interesting. As one review put it:

The flexibility and freedom in Stardew Valley is without a doubt its greatest strength. It is one of those games that gives you so much autonomy that it ends up becoming more of a roleplaying game than anything else. Your avatar in the game adopts a personality through your actions. Are they helpful and neighbourly? Are they looking for love? Or are they driven to explore the many mysteries of the village? Worryingly, my character seemed to have become a paranoid recluse, scuttling into town only briefly to buy seeds before returning to his ramshackle farmhouse, his dog and his ever expanding patch of parsnips.

I pulled what I believe are the most efficient recipes, here are the results with some pros and cons:

Quick note: Energy and Health directly related for recipes, that is to say, if health gained goes up or down comparatively between food items, energy goes up or down as well. That's why I only evaluated for health here, because once you know the most efficient recipe for health, you also know the most efficient recipe for energy.

Maple Bars (my favorite)

• Cost: 200g
• Health: +101
• Efficiency: 2.22 gold/point of health
• Pros: Grow year round, without Greenhouse. No need to water/sprinkler/plant, just harvest every 9 days forever. Convenient! Only item on this list able to be produced in Winter.
• Cons: Priciest on this list, still better than most foods though.

Bean Hotpot

• Cost: 120g for 10
• Health: +56
• Efficiency: 0.48 gold/point of health
• Pros: Only require 1 type of ingredient, very cost efficient.
• Cons: To get recipe, you must have 7♥ relationship with Clint, only grows in spring (this matters very little once you get the Greenhouse).
• Note: Make sure to plant an even number of Bean Plants, or you will end up with extra beans!

Eggplant Parmesan

• Cost: 70g for 4
• Health: +78
• Efficiency: 0.25 gold/point of health
• Pros: Most cost efficient on this list, has defensive and mining bonuses, which is helpful since you'll be doing most of your eating in the mines, were you need those bonuses.
• Cons: 2 "dependent" ingredients, the 2 ingredients grow in different seasons, you need a 7♥ relationship with Mayor Lewis to get the recipe.

Tortillas

• Cost: 150g for 8
• Health: +22
• Efficiency: ~1
• Pros: Easy crop to grow, grows across two seasons. Having the ability to heal up small increments of health while you still remember to is nice, and not wasteful with tortillas. Cost efficient (so is everything on this list, I guess). Recipe is available very early on.
• Cons: Not much? It grows across two seasons, so it's more convenient than most crop-based foods, and it's more cost efficient than maple bars. Not much to whine about there, just nothing too special.

Read on if you want details.

Maple Bars - Health +90

Here is why I like Maple Bars: Most cooking recipes have what I will call "dependent" and "independent" requirements. Independent requirements are things like wheat flour, sugar, vinegar, etc. That is to say, things you can go out and buy any time of the year, for a fixed price. Dependent ingredients are things you harvest, like crops, gathering foods, artisan goods, animal products, or in this case, tree sap. I pick maple bars because even though they cost 200g per bar, they heal most of your health, and you can get they any time of year. If you set up a sizeable tree farm for your maples, you can easily get 30 jars(just an example), (and bars) every 9 days(the harvest cycle for maple tapping), which at a cost of 6000 per cycle, isn't too bad. Depending on how you evaluate cost, these are probably one of the most cost efficient foods, and unlike most of the things on this list, the recipe is available relatively early in the game(random recipe from cooking show on television). The dependent ingredient is available year round too, which adds to it's own category of convenience. No changeover between seasons for your food source is amazingly convenient once your farm really starts to get big and you also want to spend more time away from it.

The other recipes which heal about as much either require too many dependents(crops, which adds into pricing), or require animal products(too much work, visiting coops and animals every day, feeding them every day, though later in the game they become less work), and also mostly require the same amount of independent ingredients as well.

If you want some other suggestions that might be a little cheaper, I can provide some suggestions for that too.

Bean Hotpot - Health +50

Bean Hotpot is a pretty great recipe, requiring only two green beans to make, and healing 50 health. Lets do some math to figure out how well it stacks up.

1 maple bar costs 200g, and heals 90 health, giving it a cost of 2.2 gold per point of health.

1 green bean plant costs 60g, and produces 5 green beans a season (10 days to grow, produces every 3 days, so 28-10=18, you lose the first day because that doesn't count as a growing day, so 18-1=17, and math.floor(17/3)=5), which would make 2.5 Bean Hotpots, but we'll just say 120g makes 5 Bean Hotpots, for the sake of simplicity. 5*50=250, 120/250=.48 gold per point of health.

Pretty good! The only real downside is that you only get the recipe by having a 7♥ level with Clint, so it's a little bit of work to actually get there.

Eggplant Parmesan - Health +70, Mining +1, Defense +2

This one might be a tough sell, because the two necessary crops, tomato and eggplant, grow in different seasons, so the payoff isn't immediate in terms of getting the food. At least the crops produce the same number yield(4 crops each)! That makes life much easier.

Anyways, 1 eggplant plant and 1 tomato plant make 4 eggplant parmesan, so 4 eggplant parmesan costs 70g(price of the two seeds, added), 70*4=280, 70/280=.25 gold per point of health. That is great! Plus the mining and defensive bonus, it's a pretty sweet treat, if you feel like waiting.

The wiki, on the page for Eggplant Parmesan, says it's a recipe from the cooking channel, but as the recipe masterlist states, it's actually a recipe from Lewis after 7♥, don't be fooled!

Tortillas - Health +20

Lets get down to it:

Corn seeds cost 150g, and corn plants produce (48-1-14)=33 math.floor(33/4)=8 crops per plant. it takes 1 crop to make a tortilla, so 150 gold gets you 8*20=160 health, 150/160=~1 gold per point of health. Pretty good too. Plus the recipe is available in your first fall from the cooking channel, or for purchase from the saloon, so it's not a lot of work to get.