How is US-UK airbase in Indian Ocean, hit by Iran, a strategic goldmine for US?

Diego Garcia, attacked by Iran, is a key US-UK military base in the Indian Ocean, which serves as US' primary hub to project power across Middle East, South Asia and Indo-Pacific. Often described as "unthinkable to lose", it features runways long enough for B-52 bombers and KC-135 tankers. It was a critical launchpad during Gulf War and Afghanistan war.