The point. Black has no defence against the attack along the b1-h7 diagonal.
Withdrawing the queen from the exposed d4-square before taking actions on the kingside.
Exploiting the fact that after 1...Qxc5 2.Rc1 followed by Nc7, White creates a protected square on d6.
4...e4 was preferable, securing this pawn, when 5.Rad1 followed by f2-f3 keeps pressure.