Brake hard on new cycle I saw someone after buying a new bicycle (a hybrid), pulled the brake as hard as he could for few seconds and dragged the cycle along while braking like that, 1 brake per centimeter. 

Why did he do this? I didn't had a chance to ask him but did he do this to test something or fix something? Is it usual to do this to new cycles? 

Here one more thing I should mention is that the brakes are mechanical disc brakes and before doing this to his cycle, his brake tuning was fine and after doing that, the tuning was not fine at all.
I can't add a comment yet so I provide this as an answer. The guy was bedding or burning-in the brakes. It makes sure the pads are seated and wiped of factory oil, grease, and other contaminants.

Disc brake break-in?

Sounds like he was doing it wrong. Unless the fact that the brakes weren't working after he did it indicates that the process uncovered a problem.