The Jargon File (version 4.4.7, 29 Dec 2003):

hack value
 n.

    Often adduced as the reason or motivation for expending effort toward a
    seemingly useless goal, the point being that the accomplished goal is a
    hack. For example, MacLISP had features for reading and printing Roman
    numerals, which were installed purely for hack value. See display hack
    for one method of computing hack value, but this cannot really be
    explained, only experienced. As Louis Armstrong once said when asked to
    explain jazz: ?Man, if you gotta ask you'll never know.? (Feminists please
    note Fats Waller's explanation of rhythm: ?Lady, if you got to ask, you
    ain't got it.?)


The Free On-line Dictionary of Computing (30 December 2018):

hack value

   Often adduced as the reason or motivation for expending effort
   toward a seemingly useless goal, the point being that the
   accomplished goal is a hack.  For example, MacLISP had
   features for reading and printing Roman numerals, which were
   installed purely for hack value.  See display hack for one
   method of computing hack value, but this cannot really be
   explained, only experienced.  As Louis Armstrong once said
   when asked to explain jazz: "Man, if you gotta ask you'll
   never know."  (Feminists please note Fats Waller's explanation
   of rhythm: "Lady, if you got to ask you ain't got it.")