It's not quite like writing assembly because these intrinsics are meant to translate to the "best" instructions for the particular ISA that the code is targeted

It's not quite like writing assembly because these intrinsics are meant to translate to the "best" instructions for the particular ISA that the code is targeted for. You are able to compile the same chunk of code for two different architectures, for example. If you were to write inline assembly, you would be forced to have a separate chunk of code for each architecture and surround them with #ifdefs.