We chose to use a Combat Cut rear sight placement for two reasons.
When shooting a pistol with a dot optic on it, you are focusing on the target, looking through the dot. You then superimpose the dot where you want the round to strike, and press the shot off.
With tall sights designed to co-witness inside the dots window, the rear sight being behind the optic can risk one of the two towers blocking out the dot at different angles. Now you have to spend more time to locate and utilize the dot. By mounting it in front of the optic, this possibility is totally removed.
It also provides a hard steel protective surface in front of the optic in the event that you need to perform one handed manipulations against hard or destructive surfaces like steel and concrete.