iTunes works better on a Mac than it does on Windows. There are also extra features and interoperability available when you have both a Mac and an iPad on the same account and network. For instance many of the native iOS apps have OS X partners on the Mac, and these stay synced through iCloud. The Mac version often have extra features, which allows you do some things you can't do on the iPac; but synce they sync back you can still take advantage of them.
Some examples would be:
Creating groups in Contacts.
Better management tools for Contacts.
Events in Calendars have more flexible alert and repeat options.
The iWorks apps have more layout and style features.
You can edit and sort your bookmarks for Safari more easily.
The new handoff features with iOS let you start some things on the iPad, and then continue them on the computer. Or vice-versa.
And lots of other little things that I can't think of at the moment, but would probably need immediately if they went away.
Oh, and there's an iBooks app on the Mac, so you can read your iBooks on a computer screen, if you need to for some reason.
To sum it up, it's mostly about a decent bump in reliability (iTunes mostly) and a bunch of small but useful extras.