Aside from risk mitigation there are a few other things that no one has mentioned. First, shorting. If you sell 3 month US Treasuries that you do not have becau

Aside from risk mitigation there are a few other things that no one has mentioned. First, shorting. If you sell 3 month US Treasuries that you do not have because you think that you are going to be able to buy them later at a lower price, you may have to buy them at 0% if they are called. Second, there are a number of funds that have constrained investment choice. For example, Fidelity has a short term US Treasury fund that must only invest in short term US Treasuries. And, many pension funds and government funds must invest at least a portion of their holdings in a limited set of debt holdings (sometime even further limited by credit rating or in fact by name). Finally, US Treasures can be used as collateral or for hedging as part of certain contracts and/or swap agreements.