I agree, the benefit of barrel selection is the selector knows s/he gets what s/he liked best; problem is that doesn't guarantee it is what anybody else likes best.
As cowdery stated, selection is limited to what is pre-selected /available. The key with barrel purchases is to be certain you aren't simply choosing the better of evils. It's certainly possible that all the available
selections to choose from could taste subpar to the standard brand they are supposed to represent. In such a case hopefully the selector runs instead of choosing the better of the substandards.
Another benefit I've seen bottles from barrel purchases state Non Chill Filtered and you also sometimes find the odd uncut unfiltered bottlings for retailers with special