Donuts are used in the color frame slot of an ellipsoidal reflector spotlight  to sharpen gobo projections.  Similar to the way the human eye sharpens focus when it squints, a donut reduces the aperture and sharpens the output of the fixture.  There is some reduction in brightness to achieve this increase in sharpness but this is often a tradeoff that designers are willing to make.

S4 (19-50), SL, SH, PC - 6.25"
S4 10, SL 10 - 12"
S4 5 - 14"
360Q, 6" Fresnel - 7.5"