As Neil has pointed out, the big picture is that you are part of a big electricity network, and if it wasn't grounded somewhere, the whole damn thing would float high - perhaps to $lightning volts.
Your second question "Wouldn't it be safer to just float it", becomes a very interesting question when you have a local, unconnected solar power system. The electrical regs (here) oblige you to ground N, but really, that is just making it unsaferer.
This is a topic we (installing solar power) have argued about at some length without a good conclusion.