1. You can't install Windows without a DVD drive, nor can you boot into Windows via drive sharing.
2. Ripping of music CD's, though less prevalent nowadays, still is popular.
3. Ripping DVD's.
4. Ripping Blu-Ray via external HDD
5. PC and Mac games are mostly on DVD's.
6. Adobe's Master Collection is always on DVD sets.
7. Recovery boot disks like Knoppix
8. Burning CD's and DVD's.
9. Watching DVD's rented from Redbox, or for watching hard copies of DVD's that was bought for a TV.
10. Software developers and programmers use CD's to distribute necessary drivers, kexts and programs.
11. Movies on DVD's and CD's save hard drive space.