As far as I understood, cellulose is hardly digestible for humans. It is a necessary part of our nutrition, though, because the guts (and possibly the gut flora – not sure about that) needs the “fibres”, as they tend to be called, to allow the food mass (or the stool) to better pass through the intestine.
We can therefore only digest food that doesn’t contain too much cellulose. Like fruits and vegetables, for example. Fruits and vegetables provide “fibres” that aid the digestive tract, even if the fibres themselves are hardly consumed or modified.
There’s also food that contains much more cellulose. Potatoes are a fine example, and I think beans are too. That’s why these foods are undigestable in their raw form: the cell walls contain too much cellulose, so the cells will pass through the digestive tract without their nutritious contents being available to us. These foods must therefore be cooked (or baked or whatever). Cooking causes the cellular fluid to start boiling, thereby rupturing the cellulose cell walls. This rupturing of all cell walls causes the food to become weak and, much more important, it causes the contents of the cell to be exposed so our system can digest them normally.