Using the Intel i-Core line as an example...
What is it about changing from a larger size scaling (32nm technology) to a smaller size scaling (22nm technology) that necessitates a BIOS update for IB processors to work on LGA 1155 socket 6-Series boards?
All CPUs have integrated data about itself that more or less identified the specs of the CPU. The BIOS contains a database of all CPUs that are compatible with the motherboard.
If the CPU inserted into the motherboard does not match up against that list of compatible CPUs in the BIOS, then it will not allow the PC to power up. Or if the PC may power up, then CPU is running at less than it's rated speed.