[johannes1234321]

There is a lot of indirect hardly measurable value one can gain.

Going back to the original source: By giving an answer to somebody on a Q&A site, they might be a kid learning and then building solutions I benefit from later, again. Similar with software.

And I also consider the total gain of knowledge for our society at large a gain.

While my marginal cost form many things is low. And often lower than a cost-benefit calculation.

And some Q&A questions strike a nerve and are interesting to me to answer (be it in thinking about the problem or in trying to boiling it down to a good answer), similar to open source. Some programming tasks as fun problems to solve, that's a gain, and then sharing the result cost me nothing.