ER is correct
By imaging the flow at high speed and better detail they can see how well it follows the computer models... if it matches, then GREAT the models work... if it doesn't then its cause for serious concern... because you design this thing in a way the models say is safe, if the model is wrong then you have no idea how safe it actually is.
More details here:
http://gameon.nasa.gov/2016/08/06/revolutionary-camera-recording-propulsion-data-completes-groundbreaking-test/
http://gameon.nasa.gov/files/2015/11/FS_HiDyRS-X_150504.pdf
http://www.nasa.gov/feature/revolutionary-camera-recording-propulsion-data-completes-groundbreaking-test