IP address
An IP address serves two principal functions. It identifies the host, or more specifically its network interface, and it provides the location of the host in the network, and thus the capability of establishing a path to that host. Its role has been characterized as follows: "A name indicates what we seek. An address indicates where it is. A route indicates how to get there."
The header of each IP packet contains the IP address of the sending host, and that of the destination host.
IP aliasing
IP aliasing can be used to provide multiple network addresses on a single physical interface. This demonstrates using IP version 4 addresses only. One reason for using this could be to make a computer look as though it is multiple computers, so for example you could have one server that is acting as both a gateway (router) and a DHCP server and DNS using 3 different IP addresses, perhaps with a future plan to use a hardware router and to move the functionality to separate DNS and DHCP servers. Or indeed the opposite you could decide to replace the 3 different hardware devices with a single server to reduce the administration overhead. In this case you can have 3 different addresses which are all on the same computer without having to install lots of physical network interfaces. Another reason is that you want to have the computer on two different logical network subnets whilst using a single physical interface.