It becomes a source of energy for the plants that they'll use to process many cellular activities.
Carbohydrates are generally considered the fuel for all organisms, and without this most animals would become sloth and would be unable to perform the regular jobs.
Carbohydrates actually get stored in the form of long polymers of glucose molecules or as glycogen starch. They provide energy to the animals for performing their daily duties. Carbohydrates are actually very easy to metabolise and hence generate quick energy.