Why not just up a new plane?
In the MA, mostly to not spend a sortie. If you play for score or like getting a high kills/sortie this can affect that. Sometimes you take off with too little/much gas and need to change without expending a sortie.
In the Special events, you may want to choose more or less gas depending on where the fight is. You simply can't just re -roll from the nearest base if you tower out (in the FSO for example). You have to re-arm in order to get back to the fight more quickly.
The same applies for fixing damage on the re-arm too, IMO.