Here Paul begins a two part “digression” in the flow of his argument. In light of a number of things he has said about the Law and especially in light of what he just said in the previous verses — that we have been released from the Law — Paul preemptively counters a possible conclusion some might come to, namely that the Law is sinful. But it isn’t. The Law is good, it just can’t save us. Our problem lies elsewhere.
