Let's say you're the Detroit Tigers. It's late July, you have by far the best record in baseball, and you're comfortably ahead of the second-place Chicago White Sox. You're all but guaranteed a playoff spot.
Your first baseman, Chris Shelton, has had a rough couple of months after starting the season on fire, but he still has an .819 OPS and is playing good defense. That's not bad. Your DH, Dmitri Young, is off to a bit of a rough start after losing most of the year to injury, although he has hit pretty well in the past.
So why would you be interested in Sean Casey, a first baseman who isn't a particularly good hitter for his position, is almost at injury-prone as Young is, and plays worse defense than Shelton does? Well, maybe you'd want him as a spare part - to rest Shelton occasionally against tough righties, and to DH sometimes to help keep Young fresh. If you view your current first base or DH situations as a question mark, you certainly wouldn't see Casey as a solution, since he's no better than either of the guys you currently have. He's just a good player to plug in every other day or so to keep things running smoothly. To you, Casey isn't a necessary addition; you're going to the playoffs no matter what, and Casey's not that good and is eligible for free agency at the end of the year, so anything he gives you will be a minor and temporary boost. You wouldn't mind having him, of course, but that's as far as your interest should go.
Now, if you're Dave Littlefield and you hear the Tigers might be interested in Casey, what do you do? Well, naturally, you send your scouts to take a look at Humberto Sanchez, who's one of the best pitching prospects in baseball. Surely the Tigers would be willing to trade him for Casey, right?