Because it can be used to roughly track location through 3D space. It's nowhere near as accurate as practically any other method but for "good enough" a lot of the time it'll do the job.

One I could think of is you could use accelerometer data to know if the phone is moving, and how: is it just being lifted and interacted with, is the user walking or running, are they in a car.

For a high profile target you might also be able to track their approximate location if you know their starting point and their acceleration profile - speed up, slow down, turns, time taken. Enough, at least, to execute an ambush.

> Drawing from existing literature, we found that accelerometer data alone may be sufficient to obtain information about a device holder's location, activities, health condition, body features, gender, age, personality traits, and emotional state. Acceleration signals can even be used to uniquely identify a person based on biometric movement patterns and to reconstruct sequences of text entered into a device, including passwords.

https://dl.acm.org/doi/10.1145/3309074.3309076

There was a research in 2011 about using the accelerometer to log your keystrokes https://www.extremetech.com/internet/92946-a-wiggly-approach...