The value of the use of "supra" in legal documents is the economy which is provides in citing cases which have been cited before in a document. The first time a case is mentioned, an enormously long citation must be given to its location in each of a number of reporters (books in which opinions are published). Thereafter, the case may be cited, in much shorter form, by saying "Jones, supra, at 455" which means "The Jones case, which I cited in full above, this time for the point made on page 455 of the official report."