Question

how an adaptation in angiosperms to provide nutrition for its embryos has become such a major food source for humans

Answer 1

Let's see, the nutrition storage for the embryo would be:
the endosperm in seeds 
the cotyledon(s) in seeds may also be modified for storage (and may absorb the endosperm in doing so!) 

So: we eat seeds, which includes eating the cotyledon(s) and the endosperm. Our major food sources these days that are seeds include our grain crops, the seeds of legume crop plants (e.g. beans, peas), and "nuts."