I was surprised to see that the GPS receiver I'm working with has a pin reserved for outputting a 1 PPS (Pulse Per Second) signal. What is the point of this? Can't the microcontroller easily generate its own 1 PPS signal?
The 1 PPS output has a much lower jitter than anything a MCU can do.  In some more demanding applications you can use that pulse can be used to time things very accurately.  With some scientific grade GPS's this 1 PPS output might be accurate to better than 1 nS.