It makes sense in the same way a reserve tank made sense on old VWs. Sure, you could consolidate it all into one fuel tank and get the same total capacity, but having it separate and only accessible if you flipped a switch or whatever, saves you from "oops, I wasn't watching the fuel gauge and now I'm totally out" situations.

And with modern servers able to go from "c'mon, gimme something to do" to "he's dead now!" pretty fast under the right (wrong) circumstances), you don't have to take your eye off that "fuel gauge" very long for it to count as "not watching it".

So, yeah, I can see the value to this kind of thing. Would definitely have to be documented and drilled as part of your DR/BC plan, or it wouldn't be any use. (Walk five miles to the nearest gas station, buy and fill a gas can, get someone there to give you a ride back to the car, only to be told by that person, "you do know your car has an emergency reserve tank, right?")