8120 makes more sense to me.
Useful if you do not have unlimited data plan.
Useful to avoid roaming data charges (just use your hotel's or coffee shop's wifi).
Allows you to use services like Skype (pay 2 cents per minute rather than roaming and/or long distance charges).

I use a external bluetooth GPS the rare times I need to use GPS on my phone. I would never substitute my automobile's GPS for one on a phone.