There are nice little headings in the video. But sometimes there are larger (multi-line, with paddings) headings, or simply broken ones (e.g., custom fonts with icons that don't load), and such headings are more annoying than useful to have sticky. Especially with pages that also show their headers and footers at all times, leaving little space for the content. Which makes it one of those features that should be used with care (while it is occasionally useful, and so is raising awareness about it).