The rumen has bacteria, which break down the carbohydrate called cellulose found in hay and grass. But the caecum does the same thing ,so then why is the bacteria in rumen needed?

In herbivores, the caecum stores food materials and bacteria present here breakdown the cellulose present in it.

ceacum absorbs the fluid and salts present in the digested food through its internal wall. It also lubricates food by adding mucus. Actually the digestion of cellulose and fibres are carried out by  bacteria and microbes present there. The enzymatic activities of bacteria helps in breaking down of cellulose.

The microbes in rumen of ruminent animals like cow helps in lifting milk production.

Ruminnococcus is an example of microbe that breaks plant fibre into monosaccharide glucose which is further broken down by glycolysis.