[rjbwork]

Why do you need to detect whether something is in the viewport?

[SimeVidas]

For lazyloading stuff like images and ads. Or that technique where you have an infinite list, and the elements that are scrolled off-screen are recycled on the other end.

[esailija]

You can easily scroll so that the intersection observer sentinels are not seen and cause bugs. Disappointing.

[eyelidlessness]

The linked description provides several use cases.