There are a number of reasons people might write a review:
- They are paid to do so (as per the recent case of Belkin hiring people to rate their products five star)
- They are forced to do so in order to gain some other incentive (TopTable requires you to rate restaurants you have been to in order to gain points for their loyalty scheme)
- They write reviews to increase their standing in a community (where, perhaps more reviews give them more credibility or access to more features in the online community)
- They write reviews because they want to look good / impressive / intelligent amongst their peers
- They write reviews because they had benefit from some and they want others to benefit in the same way from their advice
- They write reviews because they have something to say
This concise list looks a little frightening to book reviewers, at least at first. For instance, the concept of writing positive online reviews because somebody paid you: Many online reviewers, bloggers or otherwise, get books for free from publishers. Some see this as a form of "buying off the reviewer" (mild rant here). I occasionally get ARCs but have never written a review that didn't completely and accurately represent my opinions. Still, a few reviewers and bloggers have confessed to occasionally bluffing their reviews because they felt bad about expressing their true opinions.
Yet most of us write reviews because of that last single bullet point, which the article agrees with. Reviews are also deemed important from an economic perspective. The article goes into depth about the effectiveness and importance of leaving reviews better than I can; it's quite interesting. Ultimately, while my 3501th review of "Harry Potter and the Deathly Hallows" (I haven't reviewed it specifically) may not make a profound difference, if it gives me the feeling that it might somehow help one single reader, I've done my job.