In Japan these places are frequented during lunch, and usually right after normal working hours. People go there with some friends, and co-workers to let off the steam that has collected during the day.
Students also frequent these type of places after classes, or sporting events as well. The official drinking age is 18, but most have no trouble if they are 16, or with older friends, or dates.
The sake helps break down social barriers, and allows people to talk more freely than they normally would.