The cpu would not function without the microcode. They burn in one version at the factory, and you can update it temporarily at runtime (this update can be done both by your BIOS and by the OS), effective until a reboot.
You are using the factory-built-in version of it.
Bugfixes and possible performance improvements. What exactly is kept secret, but both Intel and AMD come with a steady stream of updates for their microcode, for the life of a cpu.
Alright, much appreciated you bothered to answer with that information.
This is disturbing. I don't like code what is kept secret, even though I would not understand a bit of the said code if it would be human readable.