Answer:
   
   Technically, you don't need one; however, it's very important to have
   a rabbi in order to make sure that the complicated marriage ceremony
   is done properly. Valid witnesses are needed to make the marriage
   official. The criteria constituting a valid witness differ among the
   movements.
   
   The purpose of a rabbi is like that of using a judge or a lawyer in
   civil matters to ensure that the law is complied with. This differs
   from the non-Jewish concept of a minister having some necessary
   mystical connection with G-d that is required to make the ceremony
   valid.
   
   In Israel, the Rav is also needed for the secular legality of the
   wedding.