If practical reason is an expression of what a person cares about, then they should contribute to society if they care about other people in society. A person may also benefit from contributing to society by improving their own social reputation.

I am a moral nihilist (error theorist) and this is what I think remains of practical reason if morality does not exist.

I think that society provides people with the means to fulfill themselves. No matter how individualistic a person is, it is society that gives them the resources for their purposes. Therefore, we must all contribute to maintaining this society.

Thanks, u/LucianoMrlli and u/AnUntimelyGuy for your replies. u/LucianoMrlli I have an addition to your point that we have to contribute to society; otherwise, we would be deemed criminals. After giving it proper thought, this is what I came up with :

As a person, we want to do things for ourselves, like a job or a business, to earn; by doing so, we provide some service (Labor or Intellect) to society, which is our contribution to society. In essence, even the job we have is for it is not for us but for the community, and we are incentivized by money to contribute to society.

People who just take from society without giving back are deemed criminals, i.e., thieves and scammers, because they do not provide anything. They are just taking from the people.

On the other hand, some people contribute to society but have different incentives, like they might want to feel good about their work or themselves.

And there would be an extreme minority of people who know they would suffer if they help others but still do it. And that is what I feel, like the people who are contributing selflessly or without taking anything from the society.

After writing this, I realize that my answer is dealing more thing the idea of the word help as help to me feels selfless.