Why do we have some network devices that have more than one IP address?

This is because a device can have more than a network interface.

Explanation:

IP address has to be unique in a network but a device can have more than a network interface. This is because the device has multiple network cards and for each of the interfaces we can set up an unique ip address in a LAN. For example almost all laptops have at least two network interfaces: one is the ethernet interface, we can connect an ethernet cable to this interface. The other one is the WiFi interface, we can connect to internet using this interface. In this case we can have more than one IP address but they have to be unique.