He has been that sort of player. Haven't you listened or read interviews from Rashford, Lingard, Welbeck in recent years? They all speak about how great Rooney has been for them and how much of a great influence he has been on them. He also practically dragged United through the 09/10 season.
As for Barkley, he barely holds his place at Everton, he's been the subject to public criticism from his own manager, has been dropped and re-instated, he could do with a wise head in Rooney to compete with him.
It's bringing a man whos won EVERYTHING, the top scorer in history for club and country, and installing him into an up and coming team. A bit like Zlatan going to PSG. Not in terms of ability but in terms of influence.