There are at least 3 reasons.
1. Not to hurt the fish during transportation, from tank to bag. You usually catch fish with your net and transport it to the container or bag right? Have you tried doing with with a pleco? Especially a pseuda pleco? Many plecos are injured while trying to release them from the net that they became tangled in.
2. Not to hurt the person during transportation. Yes, all fish struggle while they are being handled. Plecos do so with much more power. The bigger ones can really cause injury if handled incorrectly. Note the following picture, I was wearing kevlar gloves and the fish wasn't struggling. It still hurts my fingers handling them like that.
3. To display the pleco. When the plecos are handled like that, they "USUALLY" stops struggling and extends all their fins. For some of the shyer plecos, this is the only chance you'll see all their fins fully extended. Example, for L200 green phantom plecos, this is the easiest way to get them to stretch out their fins to ID which type they are.
You can't really hold other types of fish like this because they are too slippery and they don't really stop struggling when you try to do so.