You only need one because someone else uses one against you, and the law was written by lawyers in language only they can understand ;) Thus, in the hypothetical scenario of no lawyers having existed, either they would be replaced by some other ruling and representative class, or the problems they solve wouldn't have been invented in the first place.
Often, lawyers help keep someone from being against you. In contact law, lawyers are there to make keep a dispute from happening. A good lawyer will be able to foresee a potential problem and make sure you negotiate it's outcome before you enter into a deal.
