So what's the deal with spark plug washers?

They seal fine.
All you're doing is spacing the plug that much further out. Does it hurt, no.
I built my first engine as a teenager, a 225. I cam'd it, milled and ported the head, full length header...painted it chry industrial red
I NEVER REMOVED THE PLUG WASHERS.
Thing ran like a dream and always left my passengers wanting one, for the novelty I'm sure...which is what building a slant up is all about...just foolin round makin people dance with a little gun.
They make torquing spark plugs 'idiot proof' so you know ...as it crushes snug and stops...that's it!