It's true that writing software requires only computers, and that's it would be too expensive to test it in real situations, when the stakes are high maybe it's also important to do live testing ?
Hardware in the loop (HIL) is a sort of middle ground, where all the expensive stuff that breaks are replaced by ordinary computers, but the actual control hardware is not.
The mission systems avionics on a F/A-18F costs around $1-2m per aircraft. This includes Displays, PowerPC based AMC boards, power supplies etc. Hardware in the Loop testing quite practical and was ran as an engineering flight simulator. Missions were flown in the simulator, and problems located without costing $20k/hr per aircraft for an actual flight test.
