Couldn't this be just a side effect?
Surely not all not all the sensations of the human body (or any other body) are the outcome of evolutionary selection.
It doesn't seem surprising that the body experiences particular, unusual, sensations when new skin or scar tissue is being grown. Doesn't new tissue contain new nerves? At least there must be some metabolic mechanism that "connects" the newly grown tissue with the nervous system, so that you would feel a painful sensation if someone pinched your (freshly grown) scar tissue.
What sensation does the body experience when that "connection" begins to be made?
Maybe the metabolic processes by which the scab begins to separate from the wound when the healing is nearly complete (which is often the point at which the itching starts) produce the itching sensation.
Whatever the bio-mechanical origin of the itching, it is maybe just a side-effect of the repair process that serves no purpose of itself, but that is just part and parcel of a bodily repair system that obviously HAS evolved to high degree of sophistication to perform a task that confers major gene-survival advantages.