Why do i? Well. Sometimes when i write an email, i write it with a
one-draft aesthetic, much as i might if i hand-wrote it, but faster. I'll go back to correct errors, but often i find the
flow of ideas or thoughts makes something that would be better off not tampered with, even if i have something to add.
P.S.s end up being the flotsam of thought, parenthetical not to any particular bit of the message proper but to the communication itself. It's the odds-n-ends bin, and also conveys a bit of breathless last-minute addition: P.S. and P.S.S.! and P.S.S.S.!!! (though, yes, if you're following the latin, it ought to be P.P.S., P.P.P.S. &c., see below on fluency.) P.S.s can also be little unformed thought-bits, embryonic, that wouldn't survive exposure as whole paragraphs. You wouldn't want to submit them to that! So they're slung safely under the protective belly of the email itself.
P.S. is a convention, like saying bless you when someone sneezes, or wearing a tie. Or even saying "Dear so-and-so" when they are in actuality not dear to you at all! Like any convention, it can be played with; this is part of the beauty of a fluid system of signs. Knowing its original intent, as you do, can make you a more fluent and facile player in this system, if it does not make you more rigid.
Oh, and P.S.s are also good for parting shots.