Everything we have done up to now assumes that cells actually come to equilibrium with their environment. While this is a nice state to contemplate, it is almost completely unnatural. If you think about life having to gain and lose materials, for materials to flow, then equilibrium (where there is no net movement) is just not going to support life! An organism therefore usually exists in a state of gradients so that flows can occur, both between and around its cells.
Our ideas of water potential also help explain how sap rises in a tall tree. Notice that the diagram shown here is upside down, but in western cultures we read from top down, so I'm sure you can understand the flow anyway (the big arrow helps?).
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