
Because when you’re whole job is flying vip’s around DC at night, it helps to actually fly the route you’re being trained to fly at some point.
As for why the heavily used helicopter route goes right beneath the approach path, that’s because people mapped out all the routes helicopters can fly without going through restricted airspace, and along the river is one of the most useful of them, same reason as the runway’s approach path follows the river.
Given the sudden change in course after being instructed to cross behind the CRJ, the pilot likely misidentified the plane before the CRJ in the line that was just landing as one they were being instructed to watch out for, and because they were focused on the wrong plane didn’t pick out the right one directly in front of them against the city lights. Between the limited FOV of night vision goggles and the light they needed to see not moving at all from their perspective this seems more like a demonstration of the limits of see and avoid at night than gross negligence on the part of the pilot.
All that being said, the helicopter appears to have been a hundred and fifty feet above the upper bond for that route, but when the routes only two hundred feet high to begin with you don’t exactly have very much in the way of a margin for error either way.