Your phone doesn't fully decrypt until you enter your passcode the first time after a reboot. Once you enter your passcode that first time, it is decrypted for use. This is also why biometrics don't let you unlock your phone until after that first unlock and decryption, biometrics are just a shortcut.

So if the phone isn't interacted with for a period of time, it will automatically reboot. This increases device security because it forces the device back to a fully encrypted state when unused for a while.

Police don't like this because it makes their job harder to get into a device the owner doesn't want them to have access to. It also means that the time they have to get a court to force someone to unlock a device via biometrics is much shorter since that won't work after a reboot.

If your phone is running, your decryption key is stored in memory in the phone (this is slightly oversimplified, iPhones use a special chip with an enclave to do this). After a reboot, the decryption key doesn't actually exist yet until you enter your PIN (as the PIN is part of that decryption key), so it's harder to get to by an attacker.

Reboots if phone has been disconnected from network for some arbitrary time, it is suspected to be less than 24 hours that this happens in.

This stops cops from hacking the phone using exploits etc.