if there can be a new common law, what need for the code

In the US -- except for Louisiana, which has law based on the Code Napoleon -- the "LAW" consists of a quilt of standards, each piece coming from the federal or state constitution, statutes passed by the Legislature, and the common law, which was derived from the English system. The constitutional provisions supersede all other sources of law. The Legislature has primary authority for passing new laws through statutes, and if a statute does not conflict with a constitutional provision, it controls. All the rest is law created by judicial decision, and that body of judicial decision is the "common law." Despite 150+ years of the Michigan Legislature passing statutes, statutory law only addresses a portion of what is required to govern everyday affairs and to resolve controversies. Unless and until the legislature passes a statute which addresses an issue - like, say, the year-and-a-day rule, which was created in England many a year ago - the common law governs the issue, and the judiciary continues to have the power to alter the common law. So, since the Legislature never changed the year-and-a-day rule, and the Supreme Court thought that the rule was obsolete, the Court changed it. If the Legislature wants to, it can overrule the Supreme Court by enacting a statute which reinstitutes the year-and-a-day rule.

As a matter of fact, a problem will arise in Michigan which the Legislature purposely avoids because of political considerations, and the Legislature will leave it to the Court to resolve. Or, sometimes the legislative scheme is so sloppy no one can make sense of it, and the Court must engraft provisions in its role as interpreter. Often, the Legislature later specifically enacts the provisions as interpreted by the Court. Michigan's governmental immunity act is a good example of that.