One reason to do something like this would be if you were planning to disrupt the international supply chain by perhaps putting tariffs on China, keeping local unions happy to the point where they would not jump on a chance to strike during a supply chain disruption would be an effective way to minimize the adverse effects of tariffs and wars for that matter.
You could also just read this as an appeal to union members who voted for Trump to keep them a part of the Republican coalition.
Another reason is that unions have been getting increasing support from the general population, reversing decades of declining support.