What is the purpose of this NMOS transistor on an I/O pin?
If this inside an IC it is for ESD protection and referred to as a ggNMOS device.

It is intended to breakdown (without suffering damage itself) before the gate oxide of the IC when it is subjected to ESD due to static electricity.

It relies upon a parasitic bipolar transistor being formed during the normal CMOS fabrication process.  This bipolar transistor avalanches when the voltage exceeds a safe level and dissipates the input ESD strike.