Explanation
It is the most diagnostic number available on a short-form post, because it splits one confusing outcome into two answerable questions. A low hook rate means the opening failed and nothing else about the video was ever evaluated. A high hook rate with poor completion means the opening wrote a cheque the body did not cash, which is a completely different problem with a completely different fix.
Different platforms expose this under different names and thresholds, so the absolute number is rarely comparable across accounts or tools. What is comparable is the same account's posts against each other, which is the only comparison that should change what you do next.
Treat it as a steering metric rather than a scoreboard. It tells you which of your openings to make more of, and that is the entire job.
The common mistake
Comparing your hook rate to a benchmark from someone else's account. The measurement window and the audience differ, so the only useful comparison is against your own recent posts.