Refinement or extension of leg for subtle cues - not for discipline or aids to get the horse to move - horse should be doing all movements prior to introduction to spurs. But then there's this situation...
I bought a big horse from a smaller woman who used them because she simply didn't have enough leg/strength/patience to get him to listen...she sold him to me - not because anything was wrong with him, he was just too much horse under her size-wise. He was a box truck and just wanted to eat grass all day.
Now it took me almost three years to teach him to go in a plain snaffle (was on a long shanked curb) and to listen to my leg without spurs. Not a mean bone in his body and a super attitude for anything. He was just like a freight train with no go. If a turn was coming up, you had to start turning a mile before hand just to get him around. He was what I would call a horse that was dulled out from his bit AND his spurs.
The first year I squeezed and squeezed on him with my leg and had more charlie horses than I've had regular horses in my lifetime (and that is a lot) - teaching and rewarding even the smallest amount of yield. Want to have calves, thighs and buns of steel? Forget Suzanne Somers thigh burner. Get a horse with dulled sides and mouth and work with that!