It is important to distinguish between the machine where the development is done (the host) and the machine where the program will be running (the target). These two can be the same but very often you write the program on one machine and execute it on another. And very often the target machine is not identical to the host machine, which means that the program cannot be simply copied from the host to the target. If the architecture of target and host don't match, cross-compilation is necessary.