Stomates make plants much more efficient, in terms of water use, at capturing light energy in the form of sugars. This means that stomates allow plants to grow in drier places and to grow larger and more complicated bodies. This, in turn, led eventually to the evolution of trees and the formation of forests from them.
Work Step by Step
Think about efficiency and how that increases the resources available for growth. You could use a company making and selling a product like desks as an analogy to help see this— more efficient eventually leads to a larger, more dominant company.