The decal doesn't attach itself to the objects in the map, you can group the two so you can then move the object around and have the decal follow but for something like a barrel that you can pick up while playing the game the decal effect will stay where it was projected (mid air).
Unfortunately the material editor doesn’t provide for adding things like decals to the texture layer of an object with the same power that the decal tool does (you can’t scale and place a single texture to the side of a barrel for example), there’s a decal option in the material section but it’s not the same thing.