In addition to minimizing target-side reflections, I think the main reason as far as shooting optics is concerned is for contrast with the reticle. Notice that most, if not all, red dots have a red-orange coating. That gives a slightly blue-green hue to image which the red dot is easily visible against even under relatively poor light conditions (such as very bright wash-out).
My Aimpoint T-1 clone has a green coating. Same reason, it's a red/green dot, and the coating gives the picture a nice purple-magenta tint that the green is VERY easy to see against. The red is less visible, but it gives a good 'low contrast' range for darker settings.