Why the Law?

Paul makes it very clear that the promises of God have always been the point, not the Law of Moses.  But if that's the case, what was the Law for?  Was it a trick?  Was it simply to preoccupy the Israelites' time?   Paul helps the readers understand here (and in Romans) that the Law served a good purpose in guiding us to see what a life devoted to God should look like...but also convicting us that we could never do that on our own.  Praise God that the promises come true in Jesus and the indwelling of his Spirit in us.