they were also bred as farm/work dogs, for pulling carts and such.
they are not so much agressive (towards people) as they are determined, thats why the attacks are so bad, whatever they do, they give it their all. its not just gonna bite and run away, if its gonna bite its gonna tear you up, thats how the dogs (apbt) are. if its inside and it wants outside, it will go through the wall if it has to to get there.
and the jaws do not lock - the jaws are no different than any other dog - they just have stronger jaw muscles than most dogs