Thompson made a (successful) spot start Wednesday. He was needed because the Pirates had had a doubleheader earlier in the week and had also had a rash of injuries among their starting pitchers. Despite the helpful spot start, he still needs time to develop. I'm guessing Ciriaco was on the roster because that's who the Pirates always seem to add when they're not sure what else to do with that last spot. He's been on the team for a huge portion of the season, but he never seems to play.
The key to these moves is Presley, who has at least some chance of jump-starting the Pirates offense. The Bucs have been depending on the likes of Xavier Paul for far too long.
For more on the Pirates, check out Bucs Dugout.