ISO Range

My D700 has an ISO range from 200-6400 with a boost to 25,600. I've searched the web trying to learn what the IQ effects are within that range vs. in the boosted ISOs. One thing I've learned is that there doesn't seem to be a commonly accepted definition of base ISO (200 in this case) or explanation of why manufactures provide a particular ISO range; at least I haven't found one.

I know that noise (sensor gain) will increase as ISO increases, so what's the significance of the 200-6400 range? Isn't going above 6400 simply adding more gain? Why even specify an ISO range?

Thanks,
Mike