Allaah is al-Sayyid (the Master), i.e., the Sovereign of the universe; all of creation is enslaved to Him.
Allaah is the One Who is needed in an absolute sense. No created being is independent of Him. If He did not create them, they would not exist, and if He did not sustain them after creating them, they would not remain. If He did not help them with all that they are faced with, they would have no help besides Him. So it is His right that His created beings should call Him, and none other, al-Sayyid.