Pedri plays differently. Coutinho, for example, one of his best traits is shooting from 16-20m range. When you play with Messi next to you, and this is visible with Pedri as well, you can't be ignoring him and look to attack the goal, to place a shot, or enter some combination with the striker and so on. He'll start to gesticulate, make you signs that he was asking for the ball and you ignored him, and so on.
Pedri is more like Iniesta. He likes to be a service player and doesn't shy away from playing in crowded areas, especially when Messi (another player who is awesome in crowded areas) is next to him.
Coutinho is more of a direct player who likes end to end games because they give him spaces to get his game going. And Coutinho when he has space, to get a bit of acceleration, he is at his best. Because in closed spaces he lacks that burst of pace over the first meters. And, also, his technique in short spaces when the opponents are very close to him is not that great. In comparison, this is one of Pedri's best assets, the way he handles the ball in short spaces when advancing with it in a direct, forward way is not an option.
Pedri is far more suited to a slower build-up game with possession and overloads, with quick touches to maneuver the ball in seemingly very crowded and closed zones. Similar to Messi. And Puig as well to a lesser extent.
In fewer words, Messi doesn't need a "10" or a "SS" next to him. Like Coutinho and Griezmann are. He needs a classic 9 ahead of him, two quick wingers, and at least an interior "8" or "6" supporting him centrally.