The preference has to do with choosing among mutually exclusive or competing projects. If projects are not competing it makes no difference which method is used to compare benefits and costs. If projects are mutually exclusive, perhaps because funds are limited and the selection of some projects means the rejeaction of others, then the projects have to be ranked. Ranking projects by NPV preferred for reasons is beyond the schope of this article. Additional information can be found in the capital budgeting section of any college finance textbook or more advanced books such as Captial Budgeting and long term Financing Decisions.