People do that with Plecos because when you take a Pleco out of water, it'll spread out its fins and lock them in position, displaying its colours and patterns. Since it's often difficult to take a picture of a pleco with its fins spread out while it's in the water, this makes it easier for a person to show off a pleco. For example, it's hard to appreciate a Sailfin pleco fully if it's fins aren't completely displayed.
I personally don't agree with the method, because I think it stresses the fish out. And despite being able to recover if out of the water for a long period of time, from stories I've heard, there's after effects that can shorten a fish's lifespan.