It’s a typographic convention that goes back to the early days of printing.

Why would anyone use "&c" considering that it doesn't actually save any key presses?

Keypresses is a pretty poor metric to base communication upon. It annoys me to see "w/" and "w/o" anywhere except Twitter. The meaning isn't clear to many non-native speakers, and it's jarring — we recognise the most common words by the shape of the whole word, so it's easier to read "with" and "without".

"&" is a ligature for the letters "et". A traditional way to handwrite it, other than as &, is a "crossed epsilon", something like Ɛ̸, which looks more like "et". There's a Unicode character closer to this form: 🙲. I have no preference between 🙲, &, etc. and et cetera — none is common enough that I would recognize it by shape while reading prose.

The Unicode character you suggest is closer doesn't seem to render on my Mac. & and Ɛ̸ do though.

Keypresses is a pretty good metric when doing abbreviations.

How about as a stylistic choice? There are lots of little choices like that one makes when writing that together form a person's individual writing style.

Not sure what you mean - “et cetera” is nine keystrokes.

Nonetheless, Tiro would approve. ⁊c.

Probαbly the sαme reαson some people use αlpha instead of 'a' in their comments

i would! for the succinctness of it.

It’s very old-school; check out newspapers and books printed a century or two ago.

I think he means "& Co.", "and company".