The Boston Bruins won the Presidents' Trophy two years ago, but now they are a lottery team. That is the nature of the NHL; things can change quickly if you do not manage your roster properly. Their biggest mistake was trading goalie Linus Ullmark for scraps and dragging out the Jeremy Swayman contract negotiations.
I fully expect the Bruins to be much better next year. I think Swayman will be significantly better, and for the Bruins to settle down after this chaotic year. They also have $26 million in cap space and will likely look to spend.
The Bruins need to be aggressive so they can maximize the prime of David Pastrnak, Charlie McAvoy (who will be returning from a quite gruesome shoulder injury) and Swayman. I think they will be a little more hesitant to sign players to massive free agent contracts, considering how the Elias Lindholm and Nikita Zadorov deals have looked. They also still have a massive hole at center that I think the Penguins can offer a solution to.
This trade will aid the Bruins' front line considerably, as for the Penguins, it comes with a pick swap, and they would also acquire a 2027 first-round pick (the one they acquired from Florida in the Brad Marchand trade).
Boston needs depth scoring badly, especially on the wing, and Rakell provides that. For the Penguins they get the pick swap and also get a player in Mittelstadt who is still only 26-years-old and can in theory, replace Malkin, assuming he retires after next season.