Ripping your music CD collection to digital format takes a long time, even if you have a fast computer. The biggest bottleneck, though, is only being able to rip one CD at a time. If you add more optical drives, it will reduce the bottleneck and speed up the ripping process.
Internal drives will give you the best performance, but you'll need a free drive bay and SATA connection in your computer. External USB drives won't rip as fast as the internal drives, but are easier to add to any computer. No matter which you end up with, your ripping speed will increase by a lot. You'll also want to make sure that the ripping software you use supports multiple drives. If you have Exact Audio Copy or dBpoweramp, you're all set.