One great example of function pointers is in C++ when implementing inheritance. Say you have an array of Polygons, and each one has an Area() method. It turns out that some of them are Triangles, some are Squares, and some are Hexagons. polygon[i]->Area() calls a different function for each of these, and this is implemented by storing function pointers in "vtables" for each of the Polygon objects. In C++ this happens behind the scenes (you don't program this directly), but in C (or C++ for that matter if you needed to), you can get a very similar effect by storing function pointers in structs. The linux kernel makes extensive use of this programming style in driver code.
