I use sunshades on all of my match rifles. It helps keep dust and debris off of the lens a bit (especially muzzle blast dust), as well as rain. I shot for a while without my sunshades, and between glare from varying sun angle and dust on the lenses - and the IMPOSSIBLE battle of keeping rain off of the objective lens, I went back pretty quickly.
There's a slight improvement in contrast, which can make the the image appear more clear, but it's a pretty small improvement, and it really takes side by side comparison and a careful eye to discern it.