There are people who believe human nature to be any number of things, ranging from economic self-interest to collective action for benefit of humanity to supporting life on earth. To have a rightful understanding of what it is that one serves one needs to have a full picture of what it is that one is dealing with. The rational understanding of human benefit is based on a rational understanding of what is the human being, and that is as follows:
A being that has evolved as oneself, as part of one's species, and as part of life, will have orientation toward all these things. And all of these - self-interest, humanity-interest and life-interest - are valid parts of human nature that will be found, to different extents, in different people and at different times in their lives.
Which means that a truly rational conception of human being will realize all these directions and provide a workable framework that each can be pursued, with mechanisms in place to maximize benefit and minimize destructive effects.
All of these aspects of human nature, and accompanying interests, have been described by different philosophies, each of which seeing one part of the picture and confusing it for the whole. We see self-interest dominating objectivist and capitalist philosophies, humanity-interest dominating humanitarian and socialist philosophies, and life-interest dominating environmentalism. From a rational standpoint, all these describe a part of the picture and are right to affirm that part of the picture as much as they are wrong to deny others.
Thus, the rational conception of human being will value the life-interest enough to put into place mechanisms to disincentivize blindness, short-sightedness and brainlessness that comes from inattention to natural aspect of life - practices such as burning down Amazonian rainforest to create ranches that last for two years then turn into desert, or flooding the atmosphere with gases that cause overheating and floods. Instead it will put into place mechanisms to incentivize prudence and ingenuity, such as creating technologies that maximize utility while minimizing waste and environmental destruction, and creating clean energy technologies that will give people everything they have now and more at a much lesser damage to the planet.
The rational conception of human being will also recognize the vast contributions of people who were driven, not by self-interest, but by interest in the well-being of humanity: people such as scientists and teachers. Not only is most of what business sells based on science - a humanity-oriented, government-funded, altruistically valued, endeavor - but without government science, government Interstate, government Internet and government education, the economy would be a fraction of its present size. Most of the contributors to knowledge that is at the root of what people have were interested in benefiting humanity, not in advancing self-interest, and it is important that people be reminded of this from time to time as many tend to forget.
Finally, the rational conception of human being will recognize people's right to self-definition and self-interest - what is known as personal freedom - and have a framework in which people can advance their personal freedom without killing one another.
It is in understanding these aspects and providing a framework in which they can function that true rationality can be accomplished and true benefit of humanity can actually be served.