Why does my server have a dedicated IPMI port?

This system has 2 Ethernet ports, plus an out of band management port that happens to be Ethernet.

Specificlly, my concern is that accessing IPMI via LAN1 is only feasable so long as the OS (Ubuntu in my case) is not frozen.

Ability to manage a server even if the OS is missing or broken is a feature of out of band management. Wired to a dedicated management network, it also can function even if the primary NIC or switch is dead.

Imagine a small embedded computer, say Raspberry Pi size, attached to the server's power button. Has its own CPU and NIC to provide remote management, independent of the health of the larger system. Out of band management is like that, but more integrated into the board.