My reference was specifically to "law", but maybe a small preamble is in order if you're not familiar with Lutherans.
Though following in the wake of Martin Luther's teachings, the Lutheran church was not founded by Luther. It is more a name given us. And, were it not for Calvinists calling themselves the Reformed church, we might have called ourselves Reformed Catholics. The desire is always a united church, but not at the expense of the truth.
Since all Christians use the Bible in various ways (from the Bible+Tradition stance of the RCC & Orthodox to the "scripture alone" of the Protestants to the "it's a nice book" of today's liberal American churches), the Lutherans have an "exposition" of the Bible called the Book of Concord. It is not considered an infallible book, but rather the best explanation of the Lutheran interpretation of the Bible. Those who adhere to it are called Confessional Lutherans.
I call myself a Confessinal Lutheran, but I'm not perfect, so you may find a few quirks here and there in what I say about Lutheranism.
Article VI of the Book of Concord states that God gave the Law for 3 reasons (my paraphrase):
1) For the discipline of disobedient people
2) So that we'll realize we're imperfect (sinners), and can't live up to God's expectations (the Law)
3) As a guide for believers in seeking God's will and so we are witnesses to the Gospel
Law and Gospel - Wikipedia, the free encyclopedia
[edit] In Confessional Lutheran churches, what you'll hear most emphasized WRT law is #2.