C-17 displays low-level flying capability for your enjoyment and edification.
This type of low-level flying is necessary in some combat environments to evade detection. Flying the C-130 in Alaska we found lots of valleys like the one in the video in which we could hide. Radar doesn't penetrate rocks. If it's located on a mountain top, you may be able to literally fly under the radar. Airborne radars have difficulty tracking a target close to the ground and moving among mountains. Heat signature can be muffled by the surrounding terrain. Flying at 200-300 knots, you are visible to a ground observer, presumably with a shoulder launched anti-aircraft missile, for only a few seconds. If he's not an expert operator and expecting you, he may not get a lock on the aircraft before it goes out of sight. If a fighter's radar or other sensors can lock on to the aircraft, a fast moving fighter (500-650 knots) won't follow you into a deep valley because you can maneuver out of the valley, but he can't because of his speed. I can't deny that it's also fun for the pilots to exercise their skill and judgement to successfully maneuver and navigate through a series of valleys and canyons to reach a drop zone or landing zone.