Cash management bills - treasury bills shorter than the regular weekly 13 and 26 week series - have been part of the debt landscape for at least 30 years.  They are typically used to bridge occasional funding needs between regularly scheduled auctions of longer term securities.  Back when I played with them, they were intended for institutions and hence bids were required to be for at least $1MM face - typically you bid for a few hundred mil or there was no point in playing.  Institutions like money market funds and other buyers of ultra short debt treat them as part of the funiture of the repo/reverse repo/short paper/money markets.

Dick