In pots the roots of the plants tend to grow in circles entangling themselves, often referred to as becoming root bound. This overcrowding increases the likelihood of having aeration and water stagnation issues, especially in larger pots that lack proper drainage. When the roots reach the edge of a plastic pot, they continue growing in search of more water and nutrients. This begins the process of structural damage to your plants. The roots become constricted, leading to less water and nutrient intake.
Grow bags help alleviate this issue because as the roots come in contact with the edges of the fabric pot, they sense the drier soil exposed to the air. At this point, they know they have reached their growth limit. The roots become "air pruned,". This prevents overgrowth, eliminating girdling roots (roots that damage the structure of the plant). This air pruning leads to more fibrous root systems that include feeder roots for more efficient nutrition and water uptake. Plastic potted plants encourage a few larger roots to dominate and encircle the pot, reducing the plant's ability to take in nutrients.