Why does NASA fly this 70 Year Old Plane?

Thanks for watching this video hope you enjoyed
Its important to note that NASA actually kind of has a reputation for using old air frames such as the S-3 Viking and even the U-2 but the B-57 is one of if not the oldest airframes they use its also just an interesting story.
Also I'm sorry for the Red Arrows clip that doesn't feature the right aircraft it can be hard to find good footage of some of the older aircraft.