The BASIC answer to that is that when you're separated, you're LEGALLY still married, just not living together (more than likely). Being separated is usually the first step in a divorce. Divorced is not being married anymore.
Legal separation provides protection for you finacially and so forth if you are in the process of divorce or just thinking about it. Basically...if your soon to be ex goes out and runs up 20,000 dollars in credit card debt, he can't do it in your name if you have filed a legal separation with the court.