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.