Activate Cache in Processors *confussed*

I was just wondering:

If the cache (L1&L2) of a processor are completely invisible for the
programmer, being totally controled by hardware, why does Windows (not
sure about linux or other O.S) give the option to activate these and
even more, why do you get a better performance.

Can the Cache be activated de activated? Is it just a tweak?

TIA