At least for Quora (or rather StackOverflow for me) I could understand it as satisfying a need to be in a community of experts and exchange ideas on more fringe topics. Also it‘s probably a good thing to put on your resume if you have amassed sufficient fake internet points ;)

Amazon reviews however... you‘re just a drop in the ocean and there‘s certainly no community. Thus you‘d have to pay me if you want me to engage in this.

It could also be used to improve your technical writing abilities. Doesn't really matter that you have submitted review number 5397, the effort that you put into writing a persuasive argument is valuable to you in itself.

In the case of Stack Overflow, it's a great place for organic linking to your own content.

"You can do xyz in order to accomplish this task. Source: <link to my content>"

BAM. Just translated internet points into $0.002

You'll get marked as a spammer if you do this too much or in the wrong way.

For some people, I think it satisfies a human need to sort of brag about your knowledge.

For other people, it's simply a way to market their website/product by linking to their site when answering a question.