Gary is a bird photographer and he wants to know if there is a phone that can switch between Verizon and AT&T. If Gary has an unlocked phone, he can swap out SIM cards as necessary and the iPhone will switch from carrier to carrier. If he's in good standing with his carrier he can request that they unlock it. Or he can just buy one unlocked.
Google has Project Fi, which can switch to whatever the strongest carrier is, but the iPhone can't do that.