the difference between -mtune and -march

[ali hagigat]

Why two separate options have been considered for gcc, -mtune and
-march? I wonder if anybody can give an example application of using
these options while they are different.
By -mtune the code is optimized for one specific processor, what does
this optimization mean? gcc changes the assembly instructions
specified by -march and will turn them into the processor specified by
-mtune?
So why do we need -mtune? We can compile by -march from the beginning.