Explanation
It is the most quoted short-form metric and the easiest to game, because it moves inversely with length. Cutting a video in half will usually raise its completion rate without making it better, which is why an account optimising for this number alone drifts towards content too short to say anything.
Where it is genuinely useful is as a pair with watch time. High completion and low watch time means the video was short. High completion and high watch time means it held people, and that combination is the one worth chasing.
The common mistake
Treating a rising completion rate as improvement. If your average length fell over the same period, the metric moved for mechanical reasons rather than because the content got better.