Grigg (23 touches) and Houli (18 touches) were both good in what they offered. Why recruit Grigg, simple answer is Tuck can not play until he is 40. Grigg offers everything Tucky does, and adds more speed and better kicking. Houli found some nice space and set up a few goals in that 3rd term. I can definitely see why they were both picked up.
For the record, as much as I liked Collins, he had his flaws. Grigg is an upgrade.