There are at least two possible answers to this question*:
- People help those they hardly know because there are incentives embedded in the online community (e.g. reputation, points, etc.). This approach sees people as individual, rational, resource maximizers.
- People help because it is in our evolved nature to do so. We are natural cooperators, and for many forms of group activity we do not need any incentive. This approach sees people as fundamentally social animals, with an innate desire to work collectively.

Most scholars of human behavior have spent the last twentieth century working under the first assumption that we need incentive in order to do things that have no direct benefit to us (e.g. in order to get students to fill out a survey, we need to offer them money, otherwise they would not do it). Further, in cases where incentives could not be offered, then punishments needed to be given in order to encourage collective behavior (e.g. if there was no fine for littering, more people would simply drop their trash because it is not in their best interest to walk to a trash can). This assumption is strongest in economics, but it permeates even my discipline, sociology. We also see this approach in businesses, law,
and public policy. Everyone was trying to find out how to get the incentives right.
But in the past thirty years or so, research in economics, social psychology, and evolutionary biology has led to the conclusion that we are natural cooperators. In short, in any population, there are usually more cooperators (people who need no incentive) than non-cooperators. So, given a population of users in an online community, you will always get a good number of people who are willing to simply answer questions without needing any incentive – no points, no votes, nothing**.
There is a catch however. Many natural cooperators lend a helping hand, and expect help in return. If they do not get help, they will not be so generous in the future (for those who read economics or social psychology or evolutionary psychology books, this is the “tit-for-tat” strategy).
In most online forums you have very little participation because those who are willing to write posts and comments soon find that most people simply read the posts, get the information, and do not participate. Those natural cooperators then leave.
So, in an online community, there needs to be a critical mass of cooperators to keep exchanges going.
Ironically, this leads us back to the first answer, that people are selfish and need incentives to cooperate. In order to have a stable, and vibrant online community you need to make sure to provide incentives for selfish people who only respond to “votes” or “likes” or some other form of reward. If there was some way to know the composition of the people browsing a website, one could calibrate the code to fit that composition. However, we never know, so it is always a safe decision to go ahead and give rewards. That way the cooperators will be reciprocated, and the selfish ones will get their reward.
So, like most things in life, the answer is not a simple one. The answer is that people help those they hardly know because some are selfish and selfless!
*This question was posted on Quora, and I attempted to answer it there. So, this is a repost….
**Yochai Benkler‘s The Penguin and the Leviathan gives a very good review of this literature.