Why do you have to bond the neutral and the ground wire in the main panel?
The reason the neutral and ground are separate other than at the main panel is to prevent a parallel ground path. 
The power received from the power company (in the US is 240VAC)
This is derived from a step down transformer close to you home. At this point the center tap of the transformer is grounded providing 240 v across the transformer or 120v from each end to the center tap. At the house there is another ground usually a #4 or #6 copper wire coming from local grounding electrodes the grounding electrode(s) or the grounding electrodes system is there to provide a path for ground faults not as a neutral. This also provides a ground reference for the center tapped neutral to prevent the system voltage floating above ground and creating an unsafe condition if the supply from the transformer grounding point fails, it may be several hundred feet away from the home where the service grounding electrode system is very close to the service panel. 
So the grounding electrode(s) are there to help clear a fault and provide a true ground reference.