I did used to wonder why optics have risers, but then I did some research and experienced the real reason why. And it turns out for one of my AR15's it was a necessary element of my scope. 

Get laser focused and on target quickly with this reflex red dot tube style sight from 2nd Amen.

Co-witnessing refers to the alignment of the iron sights on a firearm with the reticle (crosshairs) in a scope. This allows the shooter to use the iron sights and scope simultaneously, with the iron sights visible through the scope. Co-witnessing is useful in case the scope becomes damaged or runs out of battery, as the shooter can still use the iron sights to shoot accurately. It also allows the shooter to quickly transition between using the scope for long-range shots and the iron sights for close-range shots.