The biggest benefit is that iTunes just works better on an OS X computer. It is more reliable and faster. Explore the sync settings and you may be surprised by the new options available.
iPhotos albums, including smart albums can be set up to sync with the Photo app on both the iPad and iPhone.
iCal and Calendar can be set up to sync. If you have a MobileMe account the integration of the basic apps (Mail, Address Book, Calendar) really gets good. If not then you can set up local computer syncing under the devices Info page on iTunes.
If you use GMail or another service, you can set up those accounts in the Mac's Mail, Calendar, and Address Book programs.
If you like Pages, Numbers, and Keynote, you can now get the full versions. If you move an iPad document to the OS X version first you'l have better .doc export. Still not perfect, but better.
Bento, which is barely useful on the iPad by itself suddenly becomes much better if you get the OS X version to sync with.
Keep an eye on MobileMe, rumors suggest it is going to go free, or at least have some limited free services beyond Find my IPhone. If that happens you may find a whole new slew of ways to keep your iPad and MacBook talking to each other.
Play with the iLife suite. You'll find that they communicate well with each other.
Preview is probably the least noticed but most useful app on the computer. It will allow viewing and basic editing, annotating, of most file types.
Can't think of anything else off the top of my head. Play, have fun.
