What Makes the RQ-170 Sentinel Essential for US Stealth Reconnaissance?

The RQ-170 Stentinal “Wraith” is a large US stealthy flying wing spy drone about which little is known. The United States has a history of black project aircraft whose existence stayed secret for many years (the SR-71 Blackbird and the F-117 Nighthawk being the most famous examples). The RQ-170 is not the only secret-ish spy drone the US has; it is also known to have the Northrop Grumman RQ-180 (although, again, little is known about it).