You call the FA because the person sitting in the seat has already assumed you will swap and probably won't take NO for an answer without taking up time, causing a scene or just in general being a douche.

Now we don't need someone whom either is a seat poacher or just likes to go against the grain to get a rise telling anyone they haven't experienced any of that before.

The saying goes, all it takes is one person to ruien it for everyone!

On a short hop flight there was an open middle exit row, saw a guy ask the FA if he could sit in the exit row, she said OK, she he walked up and she told the aisle this guy was going to sit in the open seat. The new guy told aisle guy to sit in the middle as "I don't sit in middle seats". Aisle guy said no and motioned for the new guy to get in, he wouldn't, the FA just stared at aisle guy like "just take it", he did. A true ... moment witnessing that.

I've had the "hope my wife doesn't puke on you!" or "FA: He just said no, wouldn't even listen to why" or blah blah blah... No thanks..