If they are like the Aimpoint, parallax free then it is to speed up target acquisition at any range. Parallax free among other things means once zeroed the dot can appear to be located in a place other than dead center of the tube/housing/scope and if it is on target, you are on target. Standard scopes do not do this. So, when you mount the gun you do not have to get your head and eye in just the right place.

Of course, coloured dot is easier and quicker to find in low light situations and well suited to close in tactical situations. Also, black cross hairs can be lost on a dark coloured target when light starts to dim.
Next advantage is there is no eye relief constraints. You can be as near or as far from the sight as you wish.

The original Aimpoints were designed to be mounted on 20mm Anti-Aircraft cannons.