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

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.