It's so that you can run narrower bars and not be placing the stem on the upsweep of a riser, which leaves the brake lever in an un-ergonomic position. Also, assuming you run the same stem, it lowers your position and moves your shoulders forward which can centralise your weight more, depending on your set-up.
If you're running wider bars, a flat bar is not necessary for lever ergonomics.
It's so that you can run narrower bars and not be placing the stem on the upsweep of a riser, which leaves the brake lever in an un-ergonomic position. Also, assuming you run the same stem, it lowers your position and moves your shoulders forward which can centralise your weight more, depending on your set-up.
If you're running wider bars, a flat bar is not necessary for lever ergonomics.