Yes, those macros are basically casts. You don't _need_ them, but you should use them as they also check that the cast is a correct one. Because C doesn't have a build-in object system you can make invalid casts (e.g. cast a GtkButton to GtkWindow) without that the compiler tells you.
If you use those macros you will at least get runtime errors what/where it went wrong.

Not sure if this was understandable If not, please tell me...

Ah yea, and the "Development" forum is probably better suited then the "Community" forum. So here this thread should be fine