First, considering what's at stake I wold think the "why" is obvious.
Second, a refer does NOT require it's own circuit. Oddly enough the code minimum installation allows a refer to be placed on one of the two required 20A small appliance circuits.
What is written is that the refer can be on it's own 15A circuit.
Most every electrician I know of will put a full sized refer on it's own 15 or 20 amp dedicated circuit. There is really no reason not to.
To add, a refer in a kitchen does NOT require GFI protection as long as the receptacle is behind it.