3,000 - 5,000 permanent stationed military personnel and civilian employees. Also has 213,000 cubic meters of fuel storage tanks. Diego Garcia was the staging point for a lot of the US Navy's ops in the Middle East during the 1990 Gulf War. If you manage to effectively target and destroy the logistics operations of the base, it creates a tough situation for the US military in the region.
EDIT - It's also got anchorage facilities for the largest aircraft carriers in the US Navy. As mentioned above, it's a popular port for USNS ships and other non-combat vessels (hell, just look at the google maps link I provided to see how many big ships are in the lagoon).