The question was asked on this thread why water until you have water run off coming out the bottom of the pot? Well for one to make sure you have no excessive dry pockets and to help to promote greater and more evenly spread out root growth that is not bunched up in the upper and middle portion of the pot. The roots will follow the water and reach towards the source and when you have water making it's way to the bottom of the pot it causes the roots to grow down towards the bottom as well as the sides and the middle of the pot. Healthy, strong and large root balls equals good harvests.
Also to maximize the room in your pot different mediums can be placed on the top of the soil such as washed and rinsed lava rock, garden type stones and even marbles for some to help the top soil to stay moist longer in order for roots to grow up as well as down making the most of your pot size. The medium on the soil stops the top of the soil from drying up so quick, which causes the roots to shy away, and also the really dry top soil causes the impact of water to be rougher when cutting threw the water to the roots, with the medium on top the water flows over the medium allowing for a slower and wider distribution of the water rather than having the water just cut into the soil, also as the water flows threw the medium it generates small but beneficial amounts of O2 which is pulled down to the roots in the water.
Hope this helps some.