My first home computer was an Apple IIgs. It had no hard drive. You need to use a "boot disk" that loaded the operating system, and then once that was in RAM, you could swap out that disk for the one with your program on it. The OS looked a little like early MacOS; it was called ProDOS. You could technically use it to copy floppy disks (the program for that was "Copy II Plus"), but it took forever, because the copy program had to copy a chunk of the disk into RAM, then get you to swap to the target disk, write that chunk, get you to swap back to the first disk, load a new chunk, get you to swap disks again... It generally took about 40 swaps for a 3.5" high-density (by which they meant 800kb) floppy. It was incredibly tedious. If you had two disk drives, though, it could just work continuously without needing to wait for you to swap disks all the time.

It was more of a convenience thing. If you had 1 drive you had to babysit the read portion to then install the CD-R after. If you had two it was just load both and carry on for 20 minutes and come back to it.

That was just if you were burning a copy and not ripping. But you're right it wasn't necessary. I just remember more than once wanting a second drive so I didn't have to sit and wait to put the CD-R in after.

It went marginally faster