I do mental math pretty much every day. Even just going through the grocery store.
I also use things I learned from calculus and physics. Like when driving. No, not solving integrals all the time but it does teach you to focus on rates of change
Just knowing that your combo can finish an opponent can switches your situation from very passive to very aggressive. Also some jungle / gank timing require clock math.
I think most serious / pro players memorize the numbers already. But casual players like me still have do head math.
I also use things I learned from calculus and physics. Like when driving. No, not solving integrals all the time but it does teach you to focus on rates of change