There are many varieties of whisks out there. Not all of them are created to do the same job. Most common types are sauce whisks, flat whisks and balloon whisks.
Sauce whisks are primarily used for blending and emulsifying sauces. This shape allows to get into the corners of a pot more easily than a balloon whisk could.
Balloon whisks main job is to beat air into light ingredients. One of the most common uses is to beat eggs with little effort. Its large and round shape aids in mixing and combining ingredients.
Perfect for making a roux or pan gravy. The flat shape puts the whisking power at the bottom of the pan where its needed most.