God acts through messengers and those messengers are angels. For example, when the Jews went out of Egypt the clouds that went with the Jews in the desert, and many other instances, it's not God himself, (well everything is God really, but) his arms and legs so to speak. God works through Messengers.
Maybe kind of like a brain that uses neurons and nerves and limbs.
But he is really acting alone because angels don't have their own will, they are like limbs of a person, you think and your arm moves.
Depending on what your philosophy is most people would agree that everything is really God, it's just how he manifests his will. Yes, if he really wanted to he wouldn't have to create Angels, but that is how he wished it to be. Why? Ask him.
It makes more sense also logically for us Mortals to understand him in a way perhaps. On Rosh Hashanah, the new year, we crown him as our King, and a king has servants and messengers and people in the palace.
With regards to leading the Jews out of Egypt, since God cherished the Jews so much he himself led them out.