Emre Ozdenoren, Stephen Salant, and Dan Silverman write,

an agent in our model would strictly prefer to have his “cake” or paycheck doled out to him by a savings club. For if the entire amount were available, resisting spending it would deplete his scarce willpower.

The authors cite a paper by David Cutler and Edward Glaeser on the curious fact that people who are careful about health in some dimensions (such as not smoking) are often careless in others (such as overeating). The Ozdenoren, et al, paper would explain this in terms of people allocating scarce willpower differently.

Thanks to Tyler Cowen for the pointer.