Imagine iTunes as a collection of links to all the stuff that you have on your computer that can be used on the iPad. So you drag, for example, a movie into iTunes movie section and it becomes available under the movies section when you click on the iPad.
It is also the only way to get stuff on and off of your iPad.
It is also the only way to get apps outside if your iPad itself and acts as a backup of apps in case, heaven forbid, the iPad gets lost, stolen or damaged.
Welcome!