So why?
The best answer I can think is, "Why not?" Have you ever created something, like a painting or a story or an invention or music or children etc? The satisfaction it brings a person to see his/her creations and then put in motion?
When Gd created the universe each of His creations He called, "Good". So He created all this and all of us with free will and sees it go into motion with some intervention. Maybe to prove something to angels. Maybe to prove things to us and for us to worship Him like angels have been doing. It satisfies Him to see His inventions do that.