Whatever the original purpose, the purposes it actually served were: a) spawning new processes (fork() then exec()) with the spawn code implemented in user-land, thus enabling agile shell development, and b) parallelism.

I would argue that (a) was easily the most important aspect of fork(). One can always get (b) by spawning more instances of the same program and passing them whatever state is to be shared. But (a) needs a decent API and to be kernel-side if one has no fork(). Yes, posix_spawn() is better, but fork() enabled Unix to kick the can of standardizing a spawn API that doesn't suck until enough experience was had with it to make such a thing.