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.