Call back programming w.r.t 'c' only, why we should use?

The point being saying w.r.t c only, as I am more comfortable in C.

I am not expecting a example which says this is how it works ... What I am expecting is why should we use the Call back function or some say it as function pointer.

I followed many blog and stack-overflow also, but not satisfied with any of those answers.

Let's say ( I am suggesting one scenario here, like sorting thing) we should use the call back thing, where a method/function will take more time for processing.

Let's say a process is there with one thread only, and the program is doing a sorting, which will take huge time ( let's assume > 1 min ). According to huge no of bloggers here we should use the function pointer. But how it would be useful ?

Any how we are having only one Program Counter and we will get some amount of time to process this process from CPU, then how it would be useful ?

If you think some other example is there to explain the function pointer concept please provide the example.

I saw some body suggesting like, if you will use function pointer, then the result u can collect later, but this sounds really awkward ! how is this even if possible ? How can u collect something from a function after returning from there ? the function would have been destroyed right !!!

In real time people use this for any change in events, so that they can get notification...( just adding a point )

I have seen some good programmer using this function pointer, I am dying to know why would I use this , surely there is something I am missing here...

Please reply, thanks in Advance.