Here is a rough example of why you would want to use it.
If you comment out [ICODE]fflush(stdin);[/ICODE] then it skips asking for input every second one, but with it you get asked every time.
int i = 1;
printf("\n\nLoop number %d - input 'Y' or 'y' to stop\n", i);
if( input == 'y' || input == 'Y' )