What is the purpose of building the mob farm's "AFK Platform" so high?

TL;DR: Is there a technical reason for building the AFK platform for a mob farm significantly above the "spawning floor" of the farm itself?

I'm relatively new to playing Minecraft, but I started a survival world on Xbox One (not the "Xbox One Edition") and began mining a nearby cave. I'm playing on "Peaceful" right now so I can build up resources without being attacked by mobs every two seconds (cheating? maybe...), and I've got a pretty good "head start" going.

I've started watching YT videos so I can build myself a little "compound" with farms and such to harvest materials. So far, I have pens for breeding (and slaughtering) cows, sheep, pigs, and chickens. I've got a vegetable farm (I modified it to have four rows - one each for wheat, carrots, potatoes, and beetroot), a sugar farm, and a tree farm. I can't find the videos I used for the last two, but they're nothing "special". I even built a cobblestone generator in one of my mining bases. Why? I guess just to see if I could...

After several bouts of trial and error, all of it seems to be going along pretty well so far (I'm trying to look into making the tree farming a bit more automated, but I can live with it as-is for now), so I figured my next step would be to try to build a mob farm. I've watched the Basic AFK Mob Farm - Minecraft 1.14 - Basic Builds from BlendsCraftTV over and over again. The design is simple and seems fairly easy to construct. I've got some more mining to do to acquire additional materials, but I think I should be able to get everything I need pretty quickly.

Regardless, watching this video a few times raised a couple of questions for me. They're all related, but the first one is this: Why build an AFK platform so far above the mob farm (starting around 10:39 in the video)? It seems an inordinately high risk for something that's apparently intended to be a survival-friendly mob farm. I'm sure there's a reason, but my (admittedly uninformed) thought would be to build your "AFK room" either:

1. at ground level near the chest, or
2. at the top of the structure near the lever

According to the Minecraft Wiki on mob spawning:

Mobs spawn naturally within a square group of chunks centered on the player, 15×15 chunks (240×240 blocks). When there are multiple players, mobs can spawn within the given distance of any of them. However, hostile mobs (and some others) that move farther than 128 blocks from the nearest player despawn instantly, so the mob spawning area is more-or-less limited to spheres with a radius of 128 blocks, centered at each player.

I don't see anything in this that says there's a "minimum" distance, and being so far away seems, to me, to carry an added "risk" of the mobs despawning. If being further away does play into it, my thought is that one could always use the Pythagorean theorem (a²+b²=c²) to calculate a "safe" distance from the spawn area on a diagonal to build a little "hut" on the ground:

35² (the distance to the "spawning floor") + b² (the ground-level distance we're trying to find) = 125² (the "safe" spawn/despawn distance, on the diagonal). This works out to be about 120 blocks away from the center point at ground level.

If distance isn't a major factor, why not just build a room at the top of the tower, near the lever, and put in a crafting table and stuff to do while waiting? I mean, sure you're just a slab away from certain death, but you could actually be productive while you're waiting. Maybe put in some automatic furnaces for smelting glass and stuff, a cobblestone generator (I think I even saw a sand generator online somewhere). Then, when you're ready, just flip the switch again and make your way down to the chest to pick up your loot.

As I said, I have other related questions about this design and its utility, but I won't include those here to avoid cluttering the issue any further.

FYI: I created a new world in Creative Mode using the same seed as my survival world to build a "test" version without the suspended AFK platform. I enclosed the "chute" below the spawning floor in glass and built a winding staircase up to the redstone clock (which I enclosed in a stone block room instead of leaving it open). Unfortunately, I ran into an apparently "known" lighting bug (they say it's resolved, but my tests would beg to differ) that kept my spawning floor well lit instead of completely dark. After an hour or so trying to eliminate the source I finally just gave up. Still, I really thought the overall design worked really well and have some ideas for starting over with it.

Solution 1:

As mentioned in my longer answer in your other question:

Spawning rules depend on the position of a player. Mobs cannot spawn if there are any players in a radius of 128 block around the spawning spot in question. That is why you need to be at your farm for it to work at all.

You normally put your afk spot above the farm, since that brings you further away from the ground and possibly the caves below. But as long as you are more than 128 block above ground or all places that aren't in your farm, but closer than 128 blocks to you are lit up properly it does not matter if your afk spot is above/below or next to your farm.

Solution 2:

The reason is because mobs only spawn from 148 blocks away from caves, if it is closer, the mobs will spawn in the caves instead of the mob farm. There's a solution to this. You can build the farmer on the ground but only if you lit up all the caves and dark spots in a 148 blocks radius. You can do one out of the two, either build it high up or build it on the ground and lit the caves in a 148 blocks radius, it's up to you. Wish you good luck!