In certain cases, you don't want the compiler to produce optimizations. When dealing with variables that are accessible both from an interrupt servicing routine (ISR) and by the regular code, the compiler should not make any assumptions about the value of a variable from one line of code to the next. Similarly, in a multiprocessing environment, there may be other processors that have access to shared memory variables, thereby making changes to them without warning. In such cases that involve memory mapped hardware devices, you need the volatile qualifier. Volatile qualifier prevents any optimizations done on the names they qualify, making the program possible to be used in such environments.   _{Your Comment}