They give a simple visual aid when detailing all the component parts of a whole.
For instance, a pie chart could show what percentage of a whole apple pie is made up of flour, sugar, apples, shortening and cinnamon.
A pie chart can also show what you spent all your salary or allowance on last year so you can see where all the money is going. The bigger the wedge, the larger portion of the total.
Here's a website that explains more about them and how to create one.