To say that Hashem created something for Himself would imply that he was lacking cv”s. Diroh b’tachtonim is a method of imparting shlaimus, godly perfection onto people. The mesilas yeshorim says in the first few paragraphs that Hashem created us to bestow upon us pleasure, and the place of that pleasure is mainly olam haba.
Hashem created us for us, to give us. We gain that by being selfless and by doing things for the sake of heaven, but Hashem does not need our avodah, does not need anything, because he is perfect and whole in every possible way.