"Investors purchased $8B of US Treasury Bonds from the non-issuing entity."

If you're Honda, you'd prefer that the purchaser of any Honda is purchasing their Honda from Honda. Honda doesn't care about the secondary sale of any one Honda, per se, but they'd certainly care if people start opening dealerships with fleets of effectively brand new Hondas immediately next to every Honda dealership.

Additionally, every seller that was a previous long-term holder represents decreased demand for Treasuries at the primary auction. Mark Carney put it eloquently yesterday during his speech with his analogy of "taking the sign out of the window". This represents someone taking their bid out of the auction.