I think it is to not show the guy how much you like them early in the relationship. I sometime do this, but it is normally to a guy I really like, it is fun, because for me I see it as tease kinda thing. Like obviously there will be hints that I would show that I like him, and that I am only interested in him, it depends how the girl plays it really. There is a way to play hard to get, to the point you plan to be with the guy/girl, but honestly if they don't, then that is literally a waste of time and harsh to the other person