Legal history is an old and well-established sub-discipline, and it can definitely help create a better understanding of past societies. Most of the time, though, legal historians deal with more than just laws; the real interesting stuff in law happens when there are disagreements about how the law should be interpreted and applied, so a lot of legal historians will spend more time looking at judicial decisions (judge's rulings) and the legislative process that created the laws, rather than just combing through old statutes.

It can be, although it has certain limitations. The existence or formulation of a particular law can say a lot about a society but without proper context, it can be meaningless.

For example the prohibition of a certain act could reflect general social norms, or it could reflect the act being prevalent, or it could reflect the norms of those who created the law as opposed to society, or the specific prohibition might actually mean something deeper, and so on.