The same reason you need any business card: making yourself a bit less forgettable.

If it's one you designed and implemented yourself, it's both a business card and a portfolio.

You just answered it by yourself.

It's for showing off to fellow nerds, obviously.

An interesting application of this could be to share your PGP/whatever public key. Anything providing storage would work but maybe so would a full computer.

Well, it would be if anyone cared about PGP.

I mean, you never know when you'll want to recompile a kernel in a hurry, or drop to a terminal for a quick ssh into your servers