The undigested roughage, grasses, hay, and other vegetation is what burns and makes animal dung flammable.
Most people who use dried dung cakes for fuel add more dried hay to the fire to keep it burning longer.
Burning manure produces a lot of smoke, though, putting those in the home at risk for lung problems. But fermented dung produces a lot of methane gas, which makes a relatively cheap and unpolluting fuel.
Although methane can be used to produce other forms of energy like electricity, it is not as widespread in use as the more common types like coal.