The decrees are basically glorified memos. They hold little to no legal weight in and of themselves, except that they direct the agencies that work under the president's control, so it's less legally binding document and more written order from your boss. The problem is that he writes ones directing them to do illegal shit, and they just do it because they know Trump and the rest of the Republicans spent decades packing the courts to protect them when this day came, and if they packed the courts, they're sure as hell not gonna ruin all that work by properly using their congressional powers.