Two simple examples:
First, say you want to get fighters from the US to Europe. The AR divert fields might be Bangor, Maine, then St John's Newfoundland, then Keflavik, Iceland, then someplace in Ireland. You don't have to fly them point-to-point from each of these getting gas on the ground along the way. You don't even have to overfly any of them. You simply plan your route so that there is enough gas in the tanks of each fighter so that if "this" AR fails, they have the gas to turn north to whatever divert field is closest "now". So they get one long flight to Germany, rather than several shorter ones, which would add up to more time & more opportunities for an aircraft to break.
Second, let's say you have multiple bases in one Saudi Arabia, and fighters will go attack targets in Iraq -- a Desert Storm type of scenario. The fighters take off from bases fairly far south in Saudi. Shortly before going into hostile airspace, they tank up so their tanks are full. Their fuel plan is to conduct the mission in hostile territory, and tank again after returning to Saudi airspace. If that AR fails, then they divert to the farthest north (closest) friendly base; in the 99% case that the post-strike AR goes fine, then they can return to their actual base farther away. They didn't have the fuel at takeoff to fly all those miles, but because of the tanker, the mission works fine that way.
Having to defend the AR divert fields, or not (simple Atlantic crossing), isn't really related to the refueling itself.
Also, the tankers, having all kinds of capacity for fuel, can be based anywhere -- not at any of the AR divert bases. They may accompany the receiver all the way across the pond (common when dragging fighters across the Atlantic), or they may orbit in friendly airspace (a Desert Storm scenario). But they probably are NOT based at the far-north base that'd be the divert field for a fighter that couldn't AR. With their range, they can be based pretty far away just fine.