Odd bypassed resistor network in the representative circuit of an MC7805 - what does it do?
These resistors are part of the voltage feedback network. Depending on how many of these resistors are in series (meaning: shorted or not) you get a different feedback network.

The feedback network sets the output voltage. So they use this to make the different 7805, 7806, 7812 etc... regulators from the same silicon design.

Basically they're all "born" as 5 V regulators. Then  by "(laser ?) zapping" the shorts across the resistors they can be made into regulators for different output voltages.