I also don't know the theory but I'd guess part of it is that it increases loyalty? I know if I got an offer that was substantially higher than expected I'd be

I also don't know the theory but I'd guess part of it is that it increases loyalty? I know if I got an offer that was substantially higher than expected I'd be much more likely to think that the company valued me specifically/thought I was special, even if they paid everyone else the same.