One reason is so you don't then both have to shuffle. Recent flight I had 21D, spouse 21A (21C had alreayd been selected). I sat in 21D. When 21C sat down I asked if he'd be willing to swap sides, which he was. (Both B and E were empty)

Had I just sat in 21C there would have been zero inconvenience to him or me having to move, and we would have ended up with same result.

While I did what I did lest I encounter a "you poach, you move" person, there's an argument for preemptively initiating an even swap of this sort. (I would not make this point over window/aisle or any row switch.)