Having an IP address allows a device to communicate with other devices over an IP based network.
Think of an IP address much like your home address. You must have an address so others know how to reach (communicate) with you. Your home address, say 123 Main Street, is unique within your town (your local network) but there are likely others with the address of 123 Main Street in other towns (other local networks).
Note: An IP address that is assigned by a DHCP server is called a dynamic IP address. If a device does not have DHCP enabled or does not support it then the IP address must be assigned manually, in which case the IP address is called a static IP address.