"But I Say to You..."
Jesus taught in a striking, authoritative way that amazed people (7:28-29). In Mt 5:17-20 he affirmed the Law and emphasized righteousness exceeding "the scribes and Pharisees," teachers noted for rigorous obedience to every commandment -- a 'fence around the law.' The law was moral / civil / criminal, enforceable by public courts and subject of debate.
If judgment found you in the right, you were right with God. Biblical code plus tradition.
Jesus' language creates an expectation that he sets up a more rigorous fence. Then Jesus gives a series of contrasts that use that expectation but undermine and transform it. Jesus is not giving new law, rather showing us a new starting point for thought & action, a new default.