As any gardener will tell you, the pot creates a controllable environment for the plant. Allows the right balance of water and nutrition, even allows the plant to be mobile (to allow it access to brighter light or better shade or more frequent watering). And as the plant grows the gardener needs to "pot on" i.e. transfer the plant to a bigger more suitable pot (or put the plan in the ground even).
Fail to transplant for too long and the plant suffers - not enough food, not enough water (making the gardeners' job harder as the feeding and watering need to take place more and more frequently). At some point the plant adapts using the drainage holes in the pot to put forth roots to meet its needs in from the soil below. This can go on for a while but as the plant grows and develops and the roots increase then the pot breaks. Oh you can hold it together with string or twine, but the pot is broken.
As a coach I've used this metaphor with many coachees - if they are the plant, what are the nutrients they need? What environment suits them best? What in their life is the "pot"? (A job, an organization, a relationship?) Those answers help create an awareness of requirements and constraints, insights into how to prune and grow in a specific direction. And those are powerful answers to possess. For with the right environment and nutrition the plant can grow and bloom.