TL;DR: Why are most of the mob farms I've seen in tutorial videos built as towers above ground? Is it just so we can actually see the whole thing being built, or is there some sort of technical benefit to having it above ground?
I have a few questions about the mob farm as described in the Basic AFK Mob Farm – Minecraft 1.14 – Basic Builds video from BlendsCraftTV. This time, I'm wondering why all the mob farms I've seen online – even the supposedly, "survival friendly" versions – are built as these tall towers reaching into the sky far above the ground? This seems to me to introduce a lot more danger into the building process when playing in Survival Mode. I would personally think it would be better to build these mob farms by digging/mining down into the ground?
One of the reasons I wonder about this is the (supposedly resolved) lighting bug that I ran into when trying to build a test/"tweaked" version of this farm in a Creative Mode world. I would think/hope that building underground would help to minimize that kind of issue.
Secondly, depending on the answer to my question about the "AFK platform", I would think it would be an ideal design to build the Redstone clock into a small structure at ground level (maybe make this your house?), then have the mob farm underneath with a stairway leading down to the collection chest (obviously with a lot of torches along the way).
Thirdly, depending on the answer to my question about the Redstone clock, if the clock is always running, you could just make picking up your loot a part of your regular harvesting "circuit". Yes, I know you could just start the clock, go do your other harvesting, then come back and stop the clock before heading down to the "killing floor", but that seems superfluous, IMO. Also, depending on the location and layout of your other farms, you might find yourself outside the radius where mobs will start to despawn.
Finally, if you build it below ground, you'll be mining a lot of resources as you dig out the hole for the farm that you can repurpose for actually building the farm itself. That just seems like a "win-win" situation.
As I said above, the only reason I can think of for building these mob farms above ground are for showing the people watching the video what the completed design actually looks like. Obviously, it'd be pretty much impossible to actually see the whole thing if it were all buried underground unless the area around it were excavated.
I'm just trying to understand the "rules" of Minecraft and how to "get the most" out of my experience.
Best Answer
TL;DR
It is way easier to do so and is efficient enough to supply you and others with more than enough recources.
The decision comes down to a the decision "efficiency" vs. "ease of building" obviously you want your farm to work as good as possible. To achieve that you would want as much spawning inside of it as possible. Since mobs can spawn in a area of (if i remember correctly) up to 128 block away from a player and only start despawning once they are at least 24 blocks away you would want to assure, that the only space that can spawn mobs within those 128 blocks is your farm.
To achieve this above ground is quite easy: Just have your afk spot >128 block above the ground and your farm <128 Block away from your afk spot.
To achieve this underground means you can either mine out all the blocks in the 128 block radius around you {math tells me that this would mean ~2,796,202 = 128^3 * 4/3 blocks since that is the volume of a sphere} or find and light up all the caves and the surfaces in that area.
I do have to mention that to my knowledge the spawning algorithm of minecraft starts to check for spawnable spaces for mobs at the lower end of the world and from there goes upwards, meaning that in theory you would get better spawn rates at low Y levels (Check Youtubers like Ilmango for the details). But if you just want a working farm which produces more than enough for you and a hand full of friends if you stay afk there 1 hour per week then the payoff is not even remotely worth this much effort. These things only come into play if you want optimality for the sake of it.