iTunes is your portal to the iPad. Although you need to set up an iTunes account to activate your iPad, theoretically you wouldn't need it after that. The advantage to using iTunes is to have a backup for all the stuff you have on your iPad. When you hook up your iPad to iTunes, it will give you the option to pull all of the stuff you downloaded on your iPad and put that on iTunes. Use this option!
If you download all your music, videos, podcasts, apps, etc directly to your iPad and if your iPad ever fails to the point where you have to restore it, you would lose ALL of your stuff. If, however, all your stuff is sitting in iTunes on your computer, it is simply a matter of syncing your iPad and all stuff will be put back.