Do you have to have a rehearsal dinner?

It's not a thank you for helping. It's a thank you for being a part of the process. Also, if you don't have a rehearsal, then you don't have to have a rehearsal meal (it can be breakfast, brunch, lunch or dinner) However, if you have a rehearsal, you should have one.

If you're having a rehearsal, you should have a rehearsal dinner, to thank everyone for being in your wedding and for showing up and rehearsing. It can be a small, simple thing, though, if nobody's throwing the dinner for you -- you can just do pizza or something.

I thought you had to buy gifts for the bridal party as a thank you. The dinner is a thank you, too?