Why is it important to have all the materials facts before beginning the search to prepare a memorandum of law?

I guess it would be just plain stupid to try to write a Memo of Law say to support your position on a case if you did not know the facts of the case.

The answer is that the material facts will define the legal issue(s) in dispute. The material facts will be compared and contrasted to similar case law with similar facts and their respective outcomes in order to argue the desired outcome in the case at hand. The particular material facts of the case at hand may or may not fall closely to material facts found in the case law. Controlling case law with similar or identical material facts and its particular outcome generally predicts the outcome of the case at hand.

A complete question leads to a complete answer. Without all of the material facts - research will usually be incomplete, or worse - inaccurate due to answering the "wrong" question. More importantly, time wasted cannot be charged to the client!