More oxygen to the roots, and the roots prune themselves before matting to the sides like they do in hard containers...you'll notice how if you let a plant go too long without water, the soil shrinks and there is air space between the soil and the side of the container, killing off those roots....that doesn't happen in a felt bag or it happens to a much lesser degree. Root pruning helps turn the roots back towards the center of the container instead of matting up against the sides becoming practically useless. I noticed a big difference in plant health going from 5 gallon buckets to 5 gallon felt bags.