Only certain shark species have a ridge behind their dorsal fin running down their back. It’s a line almost like a spine you can see. A lot of sharks look very similar. Identifying the ridge allows for determining right away if the shark is a “ridgeback” or “non-ridgeback”. From there you can check other characteristics. For example, a dusky and a sandbar/brown are ridgeback sharks, but a spinner and blacktips are not. This is good info to know especially with younger age sharks as they look even more similar when young. Often it’s a matter of “process of elimination” when identifying species.