Simple answer is, it makes your team better, they are taking a chance that it makes the team better now, and in the future. Why trade a #1 pick, Dubinsky and Anisimov for Rick Nash ? It makes your team better. Vanek is much more talented than Moulson. Sure they are both 30+ goal scorers, but Moulson was doing it while riding shotgun with Tavares. Vanek was able to score 30+ goals a year without a center of Tavares' caliber. Vanek can create on his own, he's a better passer than Moulson, he can kill penalties and is a threat to score shorthanded goals, just way more of an overall player than Moulson. Watch this video, Moulson can only wish he can do 90% of what Vanek can do. Only a person who only looks at stats would think that Moulson and Vanek are equal players.
http://youtu.be/mgmQesh-oQM