Is there any practical benefit of the Boeing-787 to perform near vertical takeoff?
The 787 was not designed to perform such a maneuver. Any jet airliner could do this.
What you need for such a steep climb is a good thrust to weight ratio (or enough excess speed to bleed off during the maneuver). A passenger twin-jet is designed such that it can still climb with at least 2.4% climb gradient after a single engine failure during a takeoff at the maximum takeoff weight (see e.g. this answer). When the aircraft carries no passengers and only enough fuel for a short demonstration flight, it is much lighter than that (source):




Aircraft
Operating Empty Weight
Maximum Takeoff Weight




Boeing 787-9
284,000 lb / 128,850 kg
560,000 lb / 254,011 kg




As you can see, the empty weight of a 787-9 is only about half its maximum takeoff weight. And if both engines are operating, that gives you a lot of excess performance for such a maneuver. At normal weights, the 787 could not perform such a steep climb and there is also no &quot;practical benefit&quot; in doing so since clearing terrain must still be possible after an engine failure, which results in much less climb performance.
Also note that the aircraft was not flying at a high angle of attack. It was flying at high pitch (although not nearly vertical). The angle of attack is the angle between the average chord line of the wings and the relative airflow. When climbing steeply, the angle of attack is therefore not necessarily high.