The reasons for being a buyer are pretty simple.
#1 You have a shot at the playoffs. The Pirates have been fairly injured all year and Cutch has been awful, yet the Pirates are only a couple of games out of the final playoff spot. Adding the needed pieces (mostly pitching) ought to push you over the top.
#2 You have a deep farm system. This is always an important consideration when looking to be a buyer. Do you have the pieces to lose? And right now the answer may actually be yes. Do the Pirates really need Adam Frazier, Max Moroff, and Alen Hansen?