Because the device records detailed usage information with timestamps such as device locks/unlocks, the screen lighting up and going dark, apps going in and out of focus, and in many cases details of exactly what you were doing in the app.
I followed the Murdaugh murder case a while back and that level of evidence was critical
The father killed his family and had a pretty good shot at sowing reasonable doubt until they pulled his sons phone telemetry and it showed the son unlocking his phone and checking socials at a time that conflicted with the dads story.
Phones truly are a surveillance dream. You couldn’t ask for a more invasive tracking device, and people love it. You couldn’t pry a phone away from most people these days
