It reduces the amount of light coming into the scope. This can be helpful when shooting into the general direction of the sun in the sky.

The negative is that in early morning and late evening, it will make your scope not gather light as well.

The are more of a mid morning and early evening tool. They do protect the objective lens too. I leave mine on, but would take off and throw in the pack for a late evening or early morning. Trouble is . . .that is also close to when you will need it, if you do.