At the end of a training session, when it's time to store the plane away, my instructor has me pump any remaining fuel from the tank and then run the engine until it quits in order to use any remaining fuel left in the lines and engine.

Is this what most people do and what's the purpose?

Also, is it okay to re-use the fuel pumped out? I would think it picks up air from vibration and dust from being pumped in and then out and then in again.

Thanks in advance.