First, on the cost side, I have just alluded to the idea that Treasury securities may provide money-like services to certain investors, such that removing them from the system may entail a welfare cost. Presumably, MBS are less money-like than Treasury securities, so this element of cost could be reduced when buying MBS.
19 Second, if the efficacy of Treasury purchases is diminished by the fact that many corporate borrowers already have plentiful access to low-cost funds, it is natural to focus on a sector that is more sensitive to financing costs. The housing market would seem to fit this bill.