When I optimize stuff, I just think of the SIMD instructions as a long sandwich toaster. You can have a normal toaster that makes one sandwich, or you can have a 4x toaster that makes 4 sandwiches as once. If you have a bunch of sandwiches to make, obviously you want to align your work so that you can do 4 at a time.

If you want to make 4 at a time though, you have to keep the thing fed. You need your ingredients in the cache, or you are just going to waste time finding them.