On the 737, the 15 gate is so when heavily preoccupied doing a go around in the normal landing configuration with 25 or 30 flap, you can simply retract the lever to the gate to get into the correct configuration whilst you then get on with raising gear, ensuring power and attitude are correct etc, and climbing away safely.
If you are on a single engine approach, you only use flap 15 for drag reasons. The procedure on a go around is to immediately retract to 1, then get on with all the other stuff.
The gates stop you over retracting flap at a time when you are very busy.