If you use iTunes U in order to keep up with courses and class material, there will probably come a time when you've watched everything and completed assignments. That means you probably have no need to keep it on your iPhone or iPad any longer. If you want to free up some space, you can always delete them just as easily as you downloaded them.
That's it. Deleting older titles and materials you aren't using anymore is a great way to not only keep your material organized, but free up precious storage space on your iPhone or iPad.