Plant cell walls are made up of cellulose. There’s a lot of energy stored in cellulose but humans can’t digest it and thus can’t use the energy. Instead, it passes through the human digestive tract as roughage.
You need a special digestive system to cope with cellulose. Ruminant animals have such a system. Ruminants have four “stomachs”. The first and largest of these, the rumen, contains millions of bacteria which basically ferment plant material. The bacteria partiall “digest” the cellulose for the animal. By products of the bacterial activity provide many vitamins and the bacteria themselves can also be digested for nutrients. The bacteria are so productive that a cow can live on a diet of cellulose so long as there is a little fat/oil and protein thrown in.
Cows can thus live on a diet of paper (cellulose) and chocolate (fats and proteins) as all their other vitamin needs are created by their gut bacteria.
Imagine if it worked the same way for us – I’d just eat Cadbury’s and leave the wrapper on.