Networks hold a unique place in the digital ecosystem as one of the two physical platforms over which data travels (devices are the other). While there is a lot of fear mongering about networks monitoring each packet that travels across them, the reality in most cases is much more benign.
Every time you power up a device it must uniquely identify itself to establish an address for future message delivery. For most devices, this is probably an address defined by the hardware or operating system. But an address is only useful if we can route messages to and from it, and so knowing something about what other devices are connected upstream and downstream of ours is critical.
Both data and voice networks rely on these addresses and a knowledge of how and where they connect to the larger network in order to provide interconnection to other users and the internet. In the mobile space, the network also needs to know where the device is relative to the many towers and wireless access points in order to now which tower to send messages from to guarantee they will be received. The mobile network needs to know where you are at all times in order to deliver messages to you. It does this in collaboration with the device, so location data often exists in at least two places.
Beyond knowing where devices are, the network often needs to verify whether devices are authorized to access network services to support billing and metering of services. And finally, in conjunction with the local operating system, the network can play a role in ensuring that the types of messages and services being delivered are compatible with the hardware, screens, and sensors tied to the connected device. For this, the network may need to know the device capabilities.