I bought my preamp, in part, because it offered a large, continuous range of balance adjustment. My room is asymmetrical, so that is one reason. But, seriously, are we to just assume that every recording ever made is perfectly balanced left-to-right? Heck no. Outside of classical recordings, I have heard crazy channel imbalances, which even differed from track to track and LP side to LP side. And are we also to assume that every component and pair of speakers is perfectly balanced internally? That's a big assumption. I bet there are many channel imbalances in audio gear, especially phono cartridges. So, why should we not be able to correct these imbalances and restore the true sound stage?
I think one of the reviewers at TAS refers to balance controls as "sound stage controls". 'nuff said!