Whats purpose of bonding to copper water pipes if main line feeding house is plastic then changes to copper in the rest of house??.
If copper I understand it would serve as a ground.

Thanks

Copper pipes could become energized. If they are not bonded to the service equipment and they become inadvertently energized the over current device will not trip leaving a potentially lethal situation.

Nick is right. There are appliances in all residences that use electricity and could inadvertanly be connected to the copper piping system through a shorted motor or heating element. Water heaters are the worst offenders. Dishwashers and garbage disposals, if not connected through an isolating plastic or rubber pipe can induce current onto the copper water piping, when things don't work properly.