There's an obscure use case for this called "Watching Video"
Which is a thing that happened long before Firefox 126, too. (Browsers have simply requested the screen wakelock themselves when a video was playing. So this API is mainly for use cases that aren't playing a video.)
The Wake Lock API is for other use cases, such as recipe websites, or other document for which you don't want the screen to go away / dim, the kind where you happen to need to look at the screen for long period of time without touching it/interacting w/ mouse and keyboard.
Prior to this API being introduced, websites used to play an inaudible/almost invisible looping media file to keep the screen awake. This has power usage implication, a small single digit number of watts (1 to 3.5 depending on os, hardware, mobile or not) is required to keep audio running (because of high priority threads, frequent wakeups, and simply because the audio hardware itself needs power).
One of those libraries source, for illustration: https://github.com/richtr/NoSleep.js/blob/master/src/index.j...