Why Do NIC’s Have Multiple RJ45 Ports?
LACP, VLAN, multiple IP homing, redundancy, failover.
e.g. my workplace has IBM Bladecenters. They had six ports on the back. That gives us – with a compatible switch – 6Gb/s (or 60Gb/s if we upgraded to 10Gbit), 6 separate cables that need to fail before everything falls offline (and we merely lose speed if one, two, three, four, or five cables drop), we can dedicate one cable to administrative purposes, have it bridge to another subnet (network, in effect) by having one IP on one cable and another on another cable, same for VLANs (so it can be present on our telephony VLAN while also being on the normal VLANs, so you can manage / monitor the telephony network from the server).
And when we need to move it between switches (e.g. to replace the switch) – which has happened a couple of times, we just string one of the cables to another switch, then another, then another, then another, until the whole thing has moved from one switch to another, still plugged in, still networked and it never goes offline for even a split second throughout the entire move.
In the Server world it’s often practical to have multiple NIC ports.
They are either used to connect to multiple networks (For instance a Storage Network, a DMZ for increased security, and and LAN network) but this is more often accomplished with VLANing (different conversation entirely)
The other reason is failover and load balancing. Multiple NICs are ‘bonded’ together using a load-balancing algorithm to give you better performance (2gb/s instead of 1gb/s) and failover, so if one port fails you stay online.