In fact, the distributors' customers aren't the end buyers at all. Instead, distributors cater to resellers on the GSA schedule. It is resellers that often end up with the task of providing a large number of systems with potentially customized components to the end user. Distributors give resellers access to hundreds of product lines in a single supplier. Distributors also have the logistical capacity to deliver those products very quickly.
Vendors also seek out the participation of the distributors.
By partnering with a single distributor, a computer vendor can sell through literally hundreds of resellers on the GSA schedule. This gives them much greater channel coverage in the federal market without the administrative headaches of managing that many letters of supply. It's hard enough to compete on cost as is. However, channel coverage can be just as important as price now that procurement reform has created such a free and easy federal government IT market. In other words, the more available a product is, the better it will sell.