Processor microcode is akin to processor firmware. Processors may need updates to their microcode to operate correctly. These updates fix bugs/errata that can cause anything from incorrect processing, to code and data corruption, and system lockups. Intel also packages pCode patches (pUnit firmware) along with microcode update. pCode is a critical firmware component for Intel SoC to work correctly. Hence a platform should have the ability to update these firmware in the field.
Post product launch, both the FW patches (together referred to as MCU henceforth) are often updated/released in the field as important fixes are available. Ability to update them in the field helps ensure stability and security of the platform after they are shipped.