I assumed it was a money-saving measure. If you have separate faucets you can't get warm water, only cold and/or hot. So to get warm water you're forced to fill

I assumed it was a money-saving measure. If you have separate faucets you can't get warm water, only cold and/or hot. So to get warm water you're forced to fill in the sink and make use of that, which saves both water and heating.