Answer:
One reason to update BIOS and firmware is to ensure they support new hardware like CPU model or graphic card. In some cases, the updates can also resolve overheating issue or motherboard issue.
However, this is not always the case that we must update the BIOS every time there is a new release because the latest update may cause problem to existing system as well.
As a general rule, only when there are some problems with the existing BIOS and firmware, we are only required to update them.