but why???? If i have already gotten off my ass to go throw the clothes in it and so I'm literally standing right next to it, in what universe won't I just press "start" and instead press a bunch of buttons to set up a timer?

If you want to run it overnight, or while you're at work, so it finishes as you arrive and doesn't leave the clean clothes in a clump for hours (or so it runs during cheaper power hours)

In Japan, most regions have cheaper electricity at night, sometimes at up to 50-60% discounts. That might be a factor as well.

It’s not common to have separate units here though. Much easier to setup a wash/dry cycle timer.

They also sell combo units (mostly for small apartments), so you don't actually have to move the clothes to one from the other