Sometimes the aid needed is a place. I have no idea what you mean by obligation or what axioms you base that conclusion on but it seems somewhere in all of that we disagree. I think we should provide people a place if they need it when we can afford to. I think, for one, it's cruel not to do so, but secondly it's good to promote a norm of doing so. We may be in need of a place some day (think war, climate change, economic crisis, etc.) We can't do more than we can do but I think we should do what we can to help.