Subject: Why would a sheeps digestive system digest most cellulose in the rumen?

I am asking this question to help with my Lower sixth examinations. I have 
found out that approximately 80%is digested in the rumen, none in the 
small intestine and a further 5% in the large intestine. This knowledge 
further confused me as I was of the understanding that the large intestine 
was mainly for reabsorption of water, not organic materials. Please submit 
an answer as it would be greatly beneficial to my revision studies. 

Re: Why would a sheeps digestive system digest most cellulose in the rumen?