There really is no reason to ever sign Mattingly except for the fact that the Yankees are probably doing him a favor because of who his dad is. Once a 31st overall pick who played shortstop, almost six years later he is a right fielder who has never played above Single-A sporting a sad .276 OBP and .611 OPS.
Still moves like this aren’t going to have any negative effect on the Yankees and maybe they have it in their minds that he could become a coach or a scout one day. Who knows? Until then he’ll probably sit on the Tampa or Trenton bench.