why use the "volatile" keyword

It's a bit hard to say why without referring to a specific example, however volatile is a standard C keyword and you should probaby refer to "The C Programming Language" by Kernighan and Ritchie.

However volatile is used when you want to force the compile to actually load a variable from memory.

For example if I have some code

int a = *0x80000; //Load a from memory

/*

Do stuff

*/

b = *0x80000;

I the Do stuff code did not change the memory location 0x80000 the compiler may try to optimise and say well I knew what the contents were when I read a so I'll use the value I read before.

This tends to be a problem when you are reading from something that can change such as;

a piece of hardware

or a piece of memory written to by some hardware or another piece of software (eg. foreground code and an ISR)