Re: I did not know that
This is like the "C intrinsics" for Intel CPUs; these are a special set of headers that let you use MMX, SSE (/SSE2/SSE3/etc.) while still having portable code.
Motorola did this earlier with AltiVec on PowerPC. The difference was that, whilst Intel screwed around with MMX, SSE, SSE2, etc, Motorola got AltiVec right first time. Thus a ton of software got written to use AltiVec, whilst it took ages for SSE to settle down. Even today it’s far easier to use Intel’s MKL/IPP libraries, which provide routine for useful functions optimised for every variation of Intel chip.