The benefit to 20x1 Threading, is that it is fairly common.
Certainly not all mods use it, but a good amount do, and more are being made all the time. There is also a bunch of aftermarket parts, like top caps, switches, top cap reduction caps(like 22mm to 20mm for example), and hybrid connectors, all in 20x1.
There are even a bunch of atomizers that instead of a 510, they're 20x1 and thread right onto the tube.
Then, you can use ANY combination of Switch, Tube and Top Cap, AND Atomizer, that you like.
there is no official standard mod threading, each manufacturer or maker uses what they please. But a large chunk of mods out there use 20X1, so IF there is an official size, its 20x1. Enough mods and parts exist that its a thing, and there will only be more mods and parts added to the pool.
This morning, I was running one of my GP PAPS V3 tubes and switch, with my hybrid connector and my Quasar.. eliminates the 510 top cap for a hybrid look. Also, I have been running my ELM Dripper on my 18500 GP PAPS SS tube, with my Vanilla Switch, because it just looks sharp together.. ALL stainless, except for a little brass accent on the switch.
In the end, its just a way to customize your setup. But, when new mods come out that are 20x1 threaded, you're not just getting the mod itself, you're now also getting three more pieces for your entire 20x1 collection. Also, if seperate tubes are available, you can just buy the tube and not the whole mod.
Edit- Oh, and of course.. say you buy a new mod, and you hate the switch. No problem, grab any of your other switches and pop it on. Most of the time, any 20x1 items will come with the disclaimer that even though they're 20x1, they cant guarantee universal fitment. Every now and then one particular thing might not fit on one other particular thing, but thats just here and there. My ELM Dripper threads onto every single 20x1 tube I have, EXCEPT, one end of one of my GP Lux tubes. Just a luck of the draw random thread mismatch that I could probably muscle through.. but since Ive got plenty of other tubes and drippers and caps and switches and hybrid connectors... You see my point now I imagine..
second Edit- And it just so happens, some of the most highly sought after mods, are 20x1. GP PAPs and Versa are 20x1, and it is widely agreed upon they both make the definition of a perfect bottom switch. Also... Real Caravelas are 20x1...