I don't think SLIRP was originally for palm pilots, given it was released two years before.

SLIRP was useful when you had a dial up shell, and they wouldn't give you slip or ppp; or it would cost extra. SLIRP is just a userspace program that uses the socket apis, so as long as you could run your own programs and make connections to arbitrary destinations, you could make a dial script to connect your computer up like you had a real ppp account. No incomming connections though (afaik), so you weren't really a peer on the internet, a foreshadowing of ubiquitous NAT/CGNAT perhaps.