Messi does this quite often, I think it is because he knows Dembele would further the attack and either go for a goal or pass it to someone who is free. However, Messi would rather pass to Pedri or Jordi Alba as they always give him back the ball
Wasn't the case that much in recent games. But rather the opposite, Messi tried and worked hard to keep up with Dembele, which is damn hard for a 33-34-year-old player. Dembele has freakish speed and acceleration.
Dembele also needs to use his speed in situations where forcing a 1 vs 1 duel is a low-risk and high-reward case. His judgment of when to push, when to get involved in dribbling sequences, and when to play it safe, keep the flow, has been the best thing about him this season. Decision making on the ball was near Neymar levels for a few games. Like a speedier, less polished version of Neymar.