Generally speaking, most ceiling fans only move air in a downward motion. This is great for hotter months since it forces the hot air downwards, which then mixes with cool air, forcing the cool air to move throughout the room. A reversible motor allows you to change the direction of the fan for use in the wintertime. This pulls cold air upwards, which then forces the warm air near the ceiling down. This allows for more even heating and has even been shown to cut heating costs by quite a bit.