Origin: The old man in the story A Very Old Man with Enormous Wings by Gabriel García Márquez simply appears out of nowhere. The wings and his mysterious nature suggest that he is an angel. Angels, within the Roman Catholic context of the story, originate from God. The old couples' poverty and their sick child possibly elicit God's pity and thus his sending the angel.
Use: the very presence of the angel has solved the couple's financial problems. But more importantly, within the Christian context of the story, thinking about "using" or "exploiting" an angel, who is a being of pure goodness sent from God, is really evil. In fact, the angel's main use is to show us that the couple and the village has become coarsened by circumstances and moral weakness, and that even though they may be nominally Christian, the true spirit of Christianity, which would lead to their caring for the old man, is absent, and has been replaced by cruelty and avarice. How they should "use" the angel is to make him comfortable and care for him, and perhaps bring the sick and unhappy into his presence so that his goodness could offer consolation and comfort to them.