Chris,
The bees need water for brooding if nectar is not available in sufficient quantity. That is where the practice of feeding 1:1 as an early spring feed to stimulate early brooding has come from. I have, in recent years, supplied them with less and less concentrated sugar solution, until it is practically only water; they still take it.
Think about it - if they are brooding heavily and there is a week of really freezing weather, all the external sources could be solid and mostly unavailable to the bees. I have started them with some sugar solution and continue with virtually water.
Dartingtons get an 'internal' entrance feeder and the rest get the entrance feeder from the outside. I have not yet experienced it so cold that the bees cannot get out for water since doing this but the time will come... I have never applied this to all my colonies - just the few that start brooding early (Dartingtons are superb for this).
RAB