1. c4 b5!?

What's the point about this?

It is a troll opening

Jaenisch Gambit.

It is similar to the Wing Gambit against the Sicilian.  The idea is to deflect the c-pawn away from influencing the d5-square.

Ah yes, I've encountered wing gambit a few times.

Isn't 1.c4 b5 a bit worse than wing gambit because in c4 b5, it is black that sacrifices a pawn for the same idea, while in Sicilian wing gambit white sacrifices it?

Correct.  The Wing Gambit is not especially good, so the loss of a tempo certainly does not make it better.  In fact, after cxb5, White is close to winning already.

Yeah, though I have to admit that wing gambit gives something for the pawn, and I feel the pressure even when I play ok against it. The games tend to be interesting.

This is actually  a pretty good gambit for begginners

Example above

Another Example

Proves this gambit is actually pretty viscous

You can play 1. g4 against 400-rated players and have games that look crushing.  That does not make it a good opening, nor even a good opening for beginners.