If you ask this question, I suppose you're among the happy persons who have never read comments on YouTube or any social media.
Users are encouraged to contribute (no matter how, would it be by gaining reputation/points, or receiving an e-mail “Hey, there is someone who needs your help on an article you purchased recently,” or anything else),
They do not have a direct and clear feedback that unhelpful comments are not welcome,
they would put comments even when they have nothing to say.
Therefore, in order to avoid useless answers:
Reduce the positive effect of any answer,
Increase the negative effect of a bad answer.
Here's an example I know well, being an active contributor: Google Maps. On Google Maps, you can post photos of touristic locations. When you post a photo, you receive:
- An immediate reward—the points you accumulate by contributing to the system, and a very nice message thanking you for your contribution.
- A long-term reward in a form of e-mails from Google, telling how much you contributed and how much persons have seen your photos.
What happens when you post a photo of a wrong location, or a low quality photo or a photo of your child/cat/whatever?
- Possibly, someone, someday, will flag it, and it will be removed. I'm not sure you are notified that your photo was removed; possibly not.
Even worse, if you post a photo of a landmark for which there are already hundreds of photos better than yours, you still get the incentive, while taking no risks of getting a negative feedback of your photo being potentially removed.
This creates a strong incentive to post photos, good or bad. And so, every popular landmark is filled with hundreds of photos which are terrible and don't bring anything to Google Maps, or plainly wrong.
Now take Stack Exchange. When you post an answer, if and only if the answer is good, you get:
- The incentive in a form of reputation points.
If the answer is useless, you get a clear and direct feedback which can take three forms:
- Negative comments.
- Reputation going down.
- Your answer quickly deleted.
This makes the user think twice before posting something. The system is not flawless (especially on Stack Overflow, being the most popular and attracting a lot of new users), but even there, you never get totally useless answers staying without negative feedback for a long time.