For two reasons. First, California law requires a written fee agreement for any matter that is expected to require more than $1,000 in fees.
But even more importantly, you need to understand exactly what you can expect of your lawyer and what he or she will expect of you. You should read your fee agreement carefully before signing it, preferably at home. You should sign the agreement only when you understand every clause in it.
Fee agreements are negotiable. Thus you should not hesitate to ask the prospective attorney to change, delete, or add to the agreement until you are completely satisfied with it.