Not every state recognizes legal separation so, depending on where you live, your choice may be made for you. Legal separation, where allowed, provides an alternative to divorce. You remain married and get to keep health insurance and file joint taxes, but you live apart. Many people choose legal separation over divorce when their religion prohibits divorce.
Alternatively, if you want to get remarried, you'll need to get a legal divorce. While you cannot transfer a legal separation into a divorce, you will have done much of the legwork negotiating which property goes where during the legal separation process. So, if you later decide to get a divorce, you'll still need to go through the steps in your state, but you may be able to speed through them a little faster.