If so please explain why. What height  etc.

Sometimes used to elevate a red dot sight, providing 1/3 cowitness between irons and red dot sights.

The height of the iron sight above the rail is 1.5" and is the ideal cheek weld for the AR, so when mounting an optic that is also the desired height of the center of the optic.

There are some low mounts for optics and sometimes it's required to add a riser to get the center of the optic at the optimum 1.5" height.

With red dot sights that are setup at absolute cowitness which is at 1.5" above the rail, some people prefers the lower 1/3 cowitness for better field of view and the iron sights doesn't interfere much with the field of view, a riser or spacer are added to raise the dot sight.

There are also optic and red dot sight mounts that are already higher than the 1.5" height so a riser are not required if you use those higher mounts.