Explanation
It is the metric that exposes how expensive an off-platform click is. Every one of these platforms would rather you stayed, which is not a conspiracy but a business model, and it means a click away is the highest-friction action you can ask a viewer to take.
Because of that, comment-to-DM routes consistently outperform bio links on response rate: they cost the viewer almost nothing and keep the interaction inside the app. Measuring only link CTR will make a working funnel look broken.
The common mistake
Optimising CTR without checking what happens after the click. A high click rate onto a landing page that looks nothing like the video is a more expensive failure than a low one.