Why do Pilots Spin their head?

A few reasons.

You are foremost looking for other traffic. Your head should be on a "swivel" keeping an eye out for others. The majority of traffic is not looking up or below or for aerobatic planes unless you are in an aerobatic box.

Secondly when you do rolls or spins or inverted stuff, you must keep the horizon as your reference.

This is to give good reference as to when you are inverted and when you need to pull up or out. You have to visualize where you want to be.

It is a REALLY good indicator as to your position, especially when doing low level.

That being said, if you were to keep your head firmly still, it would be difficult due to G's pulled and it would fool your inner ear.

The inner ear is crucial to your balance and position.

I used to live in an apartment right by the airport and I looooved it. Listening to the jets fly over at night made me fall asleep thinking about the places I'd been, and all the places I still wanted to go! Once in a while, I thought about how much it would suck if a plane came crashing through my living room, but most of the time I had happy thoughts!

when flying an aerobatic maneuver; you must have reference points on the horizon to judge your flight line. Also you use the wings and their position to the horizon to determine line of flight on the vertical. This requires that you turn your head often to check these references.

The above answer is very true, but its also the G-forces.

A zero-G aileron roll isn't so bad, but snap rolls and shoulder rolls can move your head around and point rolls are especially brutal.

keep your eyes on whats around you.