Bit of a niche, but I use inverted colors and the camera app to find ticks on my dog. Enabled by triple-clicking the lock button under Settings -> Accessibility -> Accessibility Shortcut -> Classic invert

Not technically "Accessibility" but Android has a "developer" setting called "Show Touches" which puts little dots on screen when you touch it.

I find it acts a little like being able to see the cursor on a desktop or laptop; it provides a little bit of immediate feedback that the system recognized your intent, and helps make it clear whether it's the OS or just the app when things are getting laggy.

I use the magnifier on iOS to examine small electronics on a PCB, or other small detailed things.

Similarly, on MacOS I use the Zoom feature (Command Key + mouse scroll) to pixel peep when doing graphical work.

Under "Display & Text Size": "on/off labels" and "increase contrast". "Reduce transparency" is a bit of a mixed bag but is worth trying out.

Under "Audio & Visual", "background sounds" is a built-in 'white noise' generator and can be useful for relaxing / sleeping. You wouldn't want to leave it on all the time, of course.

Under "Touch": "Back Touch" lets you map a double/triple tap on the back of the phone to various actions.

There's a lot of other neat features in iOS Accessibility (eye-tracking control! automatic captions! sound recognition!) and I'd encourage you to poke around to see what's available.

What do you mean useful to everyone?

Do you think everyone gets motion sickness?

On Android I use super dim, as someone who enjoys low light situations, even the darkest brightness setting can be too bright sometimes