A number of law departments have negotiated favorable terms with vendors. Examples include photocopy services, court reporters, travel agents, messengers, hotel chains, and similar services (See my posts of July and Aug. 2006 on various cottage industries; and of April 2, 2005 on unbundling ancillary services from law firms.).
It makes sense for a law department that has extracted favorable terms from a vendor to extend those terms to their primary law firms. More assertively, the law department might out and out require that the law firm use the designated vendor. On the order of 10 percent of the costs paid to law firms goes to disbursements, so there is room for shaving costs without nicking quality.