It is an interesting side effect of Theoretical Seating.
My experience of this is that I find myself with standby passengers taking the seat beside me. These can be staff passengers or people who have been rebooked following a misconnect.
If that person is in a group that has been spread around the cabin, there may be pressure to move to accommodate them.
I love an empty adjacent seat. I’ve never paid for one (not talking about the CE middle) so BA can do what they want with that seat. It’s not mine.