Why have humans evolved to scratch itches?

Feeling an itch is one of the ways humans and mammals have evolved to alert us to trauma occurring on the skin, such as a bite or sting. Scratching that itch would have once been a good way of removing whatever is penetrating the skin, long before tweezers were invented. Scratching also sends a signal to the brain that the bite or sting is being addressed and the itching sensation is turned off.