With the common black bladed Patridge front sight used on most BE guns, the sight can easily get lost in the black...not helped any by aging eyes! With the brightly illuminated red dot, no such problem.
Some do shoot six, sub-six or deep sub-six even with a dot but, it's not that common - strictly personal preference. The thought there is that having a relatively large "nothing to focus on" area such as the white below the bullseye, shooters are less prone to snatching at the trigger as their wobble takes them into the center of the bull.
/Bryan