The capabilities of an out-of-band management system are much, much greater than the serial console offers. BIOS and UEFI configuration are an obvious case, but you also use out-of-band management to bail yourself out of situations where the OS is unresponsive or unbootable, and to take many other management actions that would normally require physical access.

Serial consoles don't help if the box is properly wedged, or has eaten its own bootloader. With a good out of band management system, you can install an OS remotely, with disks on the session driving it appearing as local disks to the machine. Or, you can power off a box as needed, power it on, reconfigure raid cards, etc. A remote serial console is very limited in comparison.