Well, one "purpose" of an itch is to warn us that we may be near harmful plants and/or insects; or that we badly need a bath! However, I'm not sure that it's helpful to think of such features of our make up in terms of "purpose" - it's make it sound as though human beings (and all other organisms) were designed like machines, instead of evolving over millions of years. We have lots of features that serve no purpose whatever, especially those which arise after the years of procreation, so that they can have no effect on evolution - eg ear hair in older men!