This is something I've been thinking about as a DM in my encounter design: Why have easy/medium/etc combat encounters that don't really require resources to successfully complete?
Given the limited time many of our tables have to actually play, I've never created "easy" combats that don't require resources because they seem like a waste of table time. If there isn't a challenge and no resources are needed to overcome the enemy, then isn't the combat win just kind of a gimme and it's only actual resource used is everyone's actual time at the table?
I've generally looked at 5e combat like a game of resources and in order to successfully challenge my players to make it interesting, the combat generally has to be difficult – but I'm wondering if there's something I'm missing to the easy-win combats.
Best Answer
Others have mentioned other reasons, but here's another that I find particularly compelling.
In a game of D&D, it is generally accepted that the DM will build combat encounters to be difficult for the characters no matter what level they are at. This can lead to a pattern where your players feel as though, even with all the new powers they are getting, they are still weak.
Giving them a combat or two that is low level can remind the players how far they've come, especially if you choose similar enemies to a well-remembered early combat. For example, if your campaign opened on a difficult battle against a group of wolves, tossing some wolves at your now 8th-level players can help them feel as though they have grown in power.
It's a way to reward players for making progression and leveling up. It shows them that, even though on the game's main course they will always be challenged, they are still very powerful individuals, apart from the rest of the world.
It's a way to help them personally experience how far they've come, instead of just telling them so using numbers on their sheet.