They're different in different countries.

In Sweden, it's usually a time to kidnap the bride, dress her up in a funny outfit and have her do embarrassing things in town, like going up to people and asking for marriage advice while wearing a feather boa and something unusual.

It's pretty easy to spot them so the general public can guess what's going on and play along. Then it ends with a dinner or such.

I think in some places they started more as a beauty treatment preparation.

In America today, I think they're generally trying to compete with the ideas of traditional bachelor parties where the group gets drunk etc.