Possible reasons:
1.) Two different drives, 1 blue-ray reader the other a writer for example.
2.) To burn multiple CD's at the same time.
3.) Copy directly without having to copy data onto HDD.
4.) Bought new drive, and didn't bother removing old one. Or more commonly keep old drive because of a certain feature (like dvd-ram or 2x burn speed for OS .iso burning).
etc. etc.