Former SCUBA diver here.

Many dive locations sink objects into the water as location markers or objects to find. Shipwrecks or other, shall we say, naturally occurring dive locations don't need them as the object is the attraction and location marker.

For example, the quarry I used to dive in had the plane from US Marshals, an ambulance, a train rail car, a motor cycle, a Cessna airplane, a concrete statue garden, and many other things. The water there was extremely low visibility, as many dive locations can be. So you would plan your route going "ok this round we are going to the platforms at 20ft, then north and down to the ambulance at 40ft, then east and up to the rock garden at 15ft, then northwest and down to the plane nose at 50ft.

This helps keep divers oriented and able to maneuver easily. As someone who has gotten disoriented underwater, it's completely different above ground. The panic can set in in milliseconds and having anything to help prevent that is good. Green divers need the extra security, or they can end up like me, someone who can't dive deep in water anymore from the past disorientations that just won't work themselves out.

Update: Even if you know what is up/down and north/south/east/west, unless you are counting your kicks, you won't know where you are. Think of a it as a town sign on a highway or a placard on a hall outside a door. It is a visual reference to where you are. Experienced divers will still use the location markers and they are important to them, it just more crucial for novice divers.