Builds capillaries, mitochondria, and glycogen storage. This doesn't directly lead to much muscle size, but after a few weeks of doing this mutiple times you'll blow up when you return to a 6-10rep range with low volume.
I did this 3 times a week for 2 months for bi's tri's and then completely took bi/tri isolate exercises out of my routine. Only doing compound back and chest, I put 1 on my arms for the following two months and a half inch the following month.