Plants absorb more water in summer for two reasons: it is their active growing season, and they lose more water due to the heat.
Water is one of the reactants in the equation for photosynthesis; water plus carbon dioxide (plus energy from the sun) produces sugars and oxygen. This is the basic process enabling plants to grow, develop, and reproduce. Plants do more of this during the summer, and therefore use more water. During the winter, perennial plants are still alive, but relatively dormant (resting); they do not grow and develop, and need less water.
In addition to needing more water so that photosynthesis can proceed, the plants lose water due to the process of transpiration--loss of water through pores in their leaves and other above-ground parts. The plant must therefore absorb even more water to meet their photosynthesis needs.