A look at how each fits:
• Diaz is a classic "if he hits .300, he's helping you" player, with his value tied up not just in batting average but in the vagaries of his BABIP. He's overmatched as an everyday player because he doesn't hit right-handed pitching well enough, but he mashes against left-handed pitching (.335/.373/.533 career) and would work well as part of a platoon with a lefty bat. He's below-average but playable in left field, and spare parts like Diaz should have trade value in a typical July. I'm not sure why the Pirates would guarantee a second year here, though -- multiyear deals should be reserved for regulars, not bench guys.
• Correia is a back-end starter who provides value through bulk innings, mixing four pitches but without a real swing-and-miss offering. He took a big step backward in a great pitchers' environment in San Diego last year due to bad command and a big spike in his home run rate -- something that could just be a fluke. But when you work in Pitchers' Heaven and suddenly become very homer-prone, that's probably not all just bad luck.
• Olsen hasn't been the same guy since hurting his shoulder three years ago, costing him a couple mph on his fastball and a lot of bite on his slider. He doesn't command the fastball well enough (yet) to succeed as a finesse lefty, and the slider, while his best pitch, isn't sharp enough to overcome the fringy velocity. To have value to the Pirates he'll need to locate his fastball better and improve his barely-average changeup.