Seen at Anime Diet:

4. DVDs are cool, but they can be such a hassle to keep because they take up physical space. If I can download good subtitled copies and store them on my HD, I can access them so much quicker.

This is why you rip them, duh! It's the same thing as CDs and iPod. I bought a few tracks a iTunes store, but it was such an unpleasant process that my iPod ended full of CD tracks, whereas CDs themselves went to nice and compact cardboard boxes.

This also solves the backup issue heard regarding purchased tracks. CDs and DVDs are your backups. Keep them away, keep them safe.