A number of reasons. First the player. Pittsburgh's A #1 with a bullet problem last season was the power play. The coach was changed which should help, but they are still missing a right handed shot trigger man. Laine can be exactly that.
Now the plan. Pittsburgh has effectively thrown in the towel on making the playoffs next season, and on competing for at least two, if ever in the Crosby era. Laine is a boom or bust lotto ticket. If it works, you either resign him for the twilight run of Sid's career or trade him for assets before his contract expires in two seasons. If it doesn't, he's off the books before Dubas wants to make a real go at it.
With the rest, Rakell is a vanilla winger on a long term deal that might bounce back himself, might not. Either way the cap space is free when Dubas needs it. Jarry is better than Merzilkins, but again is signed longer for a little more money. The pens have options in the system so Elvis doesn't have to be the guy here. He would either be traded or bought out in two years.
I'd probably prefer to just do Laine for Rakell, but I'm willing to give to get here.