E.g. take reviews of business on Google, there's no link to actual purchases, but you get a star and a "Local guide level 4" or something if you do enough reviews. A family member runs a consulting business, he has a 2-star review, the only review. It's not made by a customer, just some random dude. What it looks like is that this dude just walked around reviewing business after business, based on look of their office perhaps. He's not customer of ANY of them. So now multiple business are trying to have these negative reviews removed, Google doesn't give a shit, so what are these reviews actually worth?
Most people who write reviews aren't exactly the most mentally stable people either. If you're not getting something in return, most people won't write a review, that just leave the nut jobs.