This article sums it up better than I could. However, I will shoehorn my two cents in.
Free will, broadly defined as the freedom to act without restraint (and/or without someone dictating your actions) is a basis for establishing moral codes. If you believe in God, you may or may not believe in Determinism -- all things are planned and you play your part according to an already-written narrative. If you are Atheist or Agnostic, you probably don't believe in Determinism, but you might believe in Fate -- "Nothing I can do about that," you say, and therefore you submit yourself to a higher power, even if that is just nature. We follow rules of society, but we are free to break them if we wish; we are not, however, free to avoid the consequences of our actions. Every action we take has an impact, either on the self or someone else, and so we must think of our Free Will not as carte blanche to act without care, but as a responsibility. We are solely responsible for our own actions, and if we reject that responsibility we descend into anarchy and chaos.