My guess: there's something out there they need the radar signatures on and they're comparing it to a known stealth signature and extrapolating. Could be an F-35. Might not be.
Or testing some sort of stealth-defeating radar technology: fly the F-117 through the sensor field, and see if the new tech picks it up well, or not. Tune. Repeat.
Meanwhile the other plane does touch-n-goes to keep the pilot up on his hours...