Cellulose is arranged into microfibrils, which give plant cells high tensile strength. This means that they are not easily stretched.
The combination of rigidity and high tensile strength provided by cellulose makes it a good structural molecule.
Cellulose is the most common organic molecule on earth (in terms of total mass). Cotton, for example, is 90% cellulose and a large proportion of leaves and wood is cellulose.
Animals are unable to digest cellulose on their own. This is because animals do not produce cellulase, an enzyme required to digest cellulose.
With the aid of microorganisms that produce cellulase, animals such as cattle (and other ruminants) and termites are able to utilise cellulose as their primary energy source. These microorganisms live in the digestive systems of such animals.