This was not the move that many expected the Blackhawks to make, but it made sense.
The Blackhawks need forward depth, as they have lost veteran forwards Brad Richards and Antoine Vermette. They have also traded young winger Brandon Saad and are expected to move forwards like Patrick Sharp, Bryan Bickell and maybe Kris Versteeg. This is why it is hard to repeat as a Stanley Cup champion, the salary cap and parity of the league makes it hard to form dynasties. This is what makes what the 'Hawks are doing even more special. Being able to win three Stanley Cups in an era where it is hard for even a defending Stanley Cup Champion to make the playoffs, and it is even harder to retain the actual thing.
In order to get forward depth, the 'Hawks have had to trade away something they haven't had in a long time. Goalie depth. The 'Hawks just recently traded away one time starter Antti Raanta to New York in exchange for a forward and now they have traded away prospect Anders Nilsson. Now the only viable goalies for the Hawks at the moment are Corey Crawford and Scott Darling. Those two alone are good enough to last a whole season but in sports things happen. The 'Hawks benefited from having three goalies last year but they won't have that luxury this year.
The player that the Blackhawks got in return for Nilsson is forward Liam Coughlin. Coughlin is still a prospect and is not projected to play in the NHL just yet. So the Blackhawks aren't getting immediate compensation, but neither will the Oilers and they don't care.
Although it could be worrying that the Hawks are essentially giving away the future to win now, the core of Jonathan Toews, Patrick Kane, and Duncan Keith is still intact. The Blackhawks always have a chance to contend if those three are playing together.