Reels are primarily used for laying guidelines, to find your way back to an anchor line, or to enter a wreck or a cave. There'd be very little reason for a brand new diver to own or use one.
Spools, on the other hand, are smaller devices that will fit in a pocket. They have a multitude of uses, including finding your way back to a starting point, sending up a surface marker (for example, when surfacing away from the boat, or in a traffic area), or serving as a jon line (a cord that allows you to do your safety stops more easily in a current). Spools are much more of a pain to wind back up than a reel is, which is why reels are used where you are going to play out a significant amount of line.
Reels are expensive; spools are not. For my money, a spool and an SMB are safety equipment, and I don't do a boat dive without them.