ive often wondered the same thing that OP did, but never 'directly' knew this, though its close to what i figured (i was figuring something about root growth). your explanation was well worded and concise, well done and a bit of +Rep to ya =DIt allows you to control the watering much better.
Imagine how long a small seedling would sit with waterlogged roots in a 5 gallon pot - it would never be able to "drink" all the water and you would be waiting forever for the soil to dry out - a recipe for disaster.