Law departments stick out for a number of reasons when compared to other corporate support groups, such as IT, HR, finance, and facilities. Eight of them occur to me, of which the first four listed below are uniquely in the legal domain.
Malpractice risks (See my post of Oct. 24, 2005: malpractice by in-house attorneys; Dec. 20, 2005: in-house indemnification policies; April 15, 2007: hold-harmless agreements in Georgia; Nov. 10, 2007: negligence and general counsel; Feb.16, 2006: malpractice insurance explains a bit why there are higher outside rates per hour; and Nov. 24, 2007: inside advantage, no malpractice premiums.).
Attorney-client privilege (See my post of Feb. 16, 2008: attorney-client privilege with 18 references.).
Make-buy balance. Other support groups buy from outside service providers, but none of them in the volume and proportion of the legal department (See my post of March 29, 2009: 40/60 ratio of inside-to-outside spend with 18 references.). Possibly IT spends a similar absolute amount, but not for the same level of services or as much on a per professional basis.
Other differences between lawyers and the remaining staff groups in a company can take the stage.
Elitism of lawyers compare to others (See my post of March 26, 2005: second-class citizens.).
Professional conflict of interest (See my post of Feb. 4, 2006: non-competes.).
Risk avoidance (See my post of Aug. 24, 2008: lawyers and risk averse behavior with 11 references.).
Officers and high pay. Law departments have a disproportionate percentage of corporate officers and a high number of employees in high pay bands