Why must my department perform a confirmation of our inventorial equipment every two years?

This is called a "Biennial Inventory," and is required by University of California policy (BFB-BUS29, Section J-1) to verify the safeguarding of inventory by Custodial departments and part of the campus' Property Control System Analysis (PCSA).  The PCSA is an external federal review of all of the campus' process associated with the acquisition, utilization and safeguarding of its inventorial property.  It is looked upon by funding agencies as confirmation that the University is judicious with the equipment it purchases from their funding.