"Sent from iPhone", and later "Sent from Mobile" were a thing because virtual keyboards were new, and at first, everyone was slow and imprecise on them.

So the signature was basically a way to say "Hey, sorry if this is abrupt or have typos, but I wrote it from my phone".

It makes zero reasons to include an automatic "Sent from <Desktop App>" signature in every email, except for sad, pathetic, marketing.