They are task loading exercises designed to teach problem solving. They also go a long way to building familiarity with ones equipment, demonstrating that most problems are not life threatening if one takes time to breathe, think, and act.
They also demonstrate an understanding of what is really important when it comes to solving problems. First is the ability to breathe, second is stability-either from the weightbelt and bc or bc if using integrated weigths, everything else is gravy once you have those two items. A diver should not have need to see to know where their releases are, how the bc goes on, etc.
And yes speaking from a purely practical mechanical point of view you may not have to do this. But from the practical side of comfort and familiarity with ones gear, the knowledge and confidence that comes from completing these tasks, which adds to the safety of the diver and those around him/her, and the ability to think through a problem under some degree of stress in an alien environment the value is priceless.
It is not always about a practical skill in the sense that you may have to do this. But once it has been done a minor issue like a backword roll and forgetting your fins (yep I did that!), having your mask come off or get knocked off, or getting the reg ripped out of your mouth by some clod swimming with their hands to who comes up behind you to see the stupid seahorse a DM is pointing at, they are indeed minor issues. Nothing to panic over, no big deal, you handled more stress than this- fix the problem and finish the dive.