I think the OP was looking for caves rather than rock formations. The difference to me is like comparing a car port to a garage.
In each tank I have some fish that swim constantly in open space, some that hang around structures, and others that like to hide in a secure place that has a single opening just big enough for them to enter.
In one tank; two peacocks, Bristlenose Pleco, and a school of clown loaches hide in caves.
Another tank it's the Spotted Raphael and a peacock taking up residence. I also have a Protomelas steveni (Taiwan Reef) who would probably move into a large cave if I had one in the tank. He's always trying to hide from the Aulonocara Firefish.
In the 150G it's the Frontosa and two Bristlenose Plecos who prefer cave life.