question about producers

[xrocketman]

what is the exact role a producer plays in making a rock album?

i know in hip-hop, producers are the ones who entirely make the beats and music,
but what does a producer do when it comes to making a rock album?

Like say George Martin and The Beatles for example. The Beatles compose their own songs, so what the hell does Mr. Martin do?

Also theres Rick Rubin with RATM's Renegades of Funk, etc.

whats the deal?

[Sandinista!]

I'm guessing they sit behind the mixing board and try create an adequate "mix" of all the various instruments, plus making sure that studio tricks sound well.