Why do you itch, and why do you need to scratch it?
It depends on where you are from. Both are are used interchangeably. But, if you must be woke then it is scratch an itch. Sometimes it has nothing to do with itching at all. It can mean something you are passionate about doing that you have not been able to do. So, you say “I’ve got an itch that really needs scratching!” And stuff like that.