I'm not sure either. My understanding is that they let you compile (part of) your program to a middle layer, a more abstract version of X86/64 instructions. And the binary then can be generated based on the actual machine using the instruction set that machine supports. So if some of the computation you do prefers a certain instruction, but you still what it to run without it, and you don't what the user to recompile from the source, then you can use this.
