Sometimes they bring in Gonzalez for key left handers. The batters due up in the 8th for the Pirates were Nyger Morgan, Delwyn Young, and Adam LaRoche. Morgan and LaRoche are both lefties and Young is a switch hitter who has been slightly better against righties this year.
Honestly, if you have Gonzalez and Soriano is available, pick him up. After today, Soriano's ERA is below 1, as is his WHIP. He gets K's at a very good rate as well.