Villa: without looking at any technical manuals, there are two things that.i can think of off the top of my head.
1. The simple answer would be tha airbus want you to do this just to maintain reletavily normal operating procedure.
2. In the event of a go around airbus might not want pilots inadvertently raising the slats into the Config 1 position due to the potential of stalling at low speeds, the lift is already already compromised by the lack of flaps, so keeping the flaps lever at Config 3 ensures that an error cannot be made.
Just some thoughts.