I covered this topic long back. I’m covering this again with a caveat!
Let’s say between the period of provisional and complete, someone else files or use the almost similar invention. Now, issue is, technically, your right to invoke full rights, retrospectively, starts post grant only! Because usually no one knows how many claims would end up getting accepted or struck down, thus, till then, one has to wait! If anywhere it happens the other way around, then, that’s a problem, because, without the grant and on the basis of contingency & unlikeliness, one cannot enjoy the fruits of something which is still abstract and hypothetical!