Zac67 and Ron Maupin have briefly explained how multiple IP addresses per NIC work, which was probably the intent of the question.
Here, I try to explain why one would add a second IP address to a host or gateway.
In most cases nowadays, one would normally use multiple VLANs for these situations, with one subnet per VLAN.
VLANs were standardised much later then Ethernet (1998 for IEEE 802.1Q compared with 1986 for IEEE 802.3), and it was not until about 2002 that Layer 3 workgroup switches became affordable. VLAN capability in branch office routers was not standard until several years later.
Therefore, the ability to bind multiple IP addresses to a single NIC without VLANs has existed for a long time in Unix and Linux, and in Windows since at least NT4.
Reason 1 : Shortage of IP addresses
Imagine you have a site with a public /24 (Class C) network (254 useable addresses) and you need more.
The easiest way to expand is to ask your ISP for another block of addresses which can access the Internet through a secondary IP address on the router (ignoring the need for a firewall for the moment).
Both subnets will be available at every port on the network, but without any other changes, all traffic from subnet B to subnet A must pass through the router, which reduces performance and adds congestion on the router port.
If you then add a secondary IP address to the server(s), PCs on subnet B can access the server(s) without passing through the router.
Reason 2 : Migration of IP adressing
When interconnecting isolated sites and new acquisitions into a corporate network (especially in a NAT environment), it's often necessary to change the subnet used.
As described in Reason 1, one can simply do this by adding a secondary IP address in the new subnet to the router and server(s) on site, then over time changing the configuration (static IP addresses, static DNS addresses, static IP addresses in applications ...) before retiring the old subnet addresses.
Note : DHCP servers or forwarders may have problems assigning IP addresses on their secondary subnet - it's better to put the new IP address as primary and the old as secondary.