The U.S. Air Force has added two more DMON (Distributed Mission Operations Network) sites for F-15E aircraft simulators. There are now ten bases with F-15 or F-16 simulators that are connected to DMON (which enables over a dozen simulators to operate together, even if they are overseas or across the country). DMON thus enables pilots to train for missions and practice cooperating with other aircraft. DMON was developed and rolled out in the 1990s, and has proven very popular and useful.
While the simulators cost nearly as much as the aircraft they simulate, they are much cheaper (less than ten percent as costly) to operate. You can also practice dangerous maneuvers, that might arise in combat, but would be too risky to practice in peacetime. The rising cost of fuel also makes the simulators more cost-effective, as a jet fighter burns over $10,000 worth of fuel an hour when performing combat maneuvers.
The simulators have become more realistic in the last two decades, as electronics (especially graphics related stuff) got cheaper and more powerful. It's now possible to do a lot more pilot training in a simulator, saving wear and tear on aircraft, and instructors.