Q:

Why is pin protection needed?

I have read this article about protecting input pins.
One thing is still unclear to me — why is the protection needed at all with such a high resistance on the input pin inside of the microcontroller? The current would be really small and the heat dissipation will be little as well.

A:

The high voltage will destroy the gate oxide of the MOS transistors connected to an input pin. The real threat is from static electricity that builds up during handling and assembly of circuit boards.

A:

Electrons are not only useful as carriers of electricity.  They are also responsible for binding atoms together in useful configurations.  Some materials have large numbers of "loose" electrons which can move around relatively freely without particularly affecting the material through which they are flowing.  Some other materials don't; given enough voltage, the electrons locked into the latter materials' structures can be knocked loose, but the act of knocking the electrons loose may permanently alter the structures in question.  A small amount of damage, concentrated in one place, can make a chip useless, and--in the absence of protection--it wouldn't take much energy to cause such damage.  The purpose of protection circuitry is to absorb energy over a much wider area, thus increasing the amount of energy that would need to be applied to cause damage.

