For one, he would fill a desperate need.
Chelsea do not have any players on the wing that can make things happen. Yes, Juan Mata can play out there, but he fits better in a more central role for Chelsea. As long as he plays his current position, the Blues need to find a new winger.
Hulk fits the bill.
His combination of pace and strength is uncommon for wingers. He can hold off stronger defenders, as well as blow by quicker ones. This combination, along with his lethal shot, makes him one of the best wingers in the world.
With Hulk on the right side, Chelsea can keep Mata in the middle, slide Ramires over to the left side and move Daniel Sturridge, Fernando Torres or another potential transfer up top. All of a sudden, Chelsea would have one of the most dynamic attacking fronts in England.
Admittedly, Chelsea still have other holes to fill. However, if Hulk solves the problem on the wing, Mata could be the answer to the creative midfielder question.
With a talent like Hulk on one side and speed like Ramires on the other, Mata will flourish in the center of Chelsea's attack.
Mata has looked exhausted this year, running from one side of the pitch to the other. With Hulk locking down one side and Ramires locking down the other, Mata will be able to sit in a withdrawn attacking role, playing long through balls and quick give-and-gos to disrupt defenses.
By bringing in Hulk, Chelsea will be able to address their biggest flaw. Likewise, the Blues will be able to field a more potent attack.
If Chelsea want to optimize their attacking positions for next year, they will add Hulk to the roster.