Food is broken down in order to gain nutrients. Simple as that.
It's done in stages because that's just the way our bodies are designed. The stages take place as the food passes through our body.
1) mouth: chewing causes release of saliva which contains enzymes to help break down food to a swallowing size!
2) stomach: food is broken down further by digestive fluids.
3) small intestine: 3 further bodily fluids are released to finalise the digestion and then the body absorbs all the nutrients.
4) large intestine: basically where anything leftover is ejected from the body, except for fibre which is stored here.
Er, that's really only a rough outline that I remember from biology classes!!
So in short, food is broken down in stages because otherwise it would simply not go into the body! Unless maybe you liquidize all your food first, but I wouldn't recommend that. :o\