CTap

Without -1's your reputation isn't properly reflected.

@Enyahs

1. Some people dont have time to reply to peoples posts.

2. If you say "Hey, your post was a waste of my reading time because dadda dadda dadda" you will look like a ****

3. Its more convenient for people to just -1 a post

4. People might not have permission to post on said topic but still want to give there opinion (-1, +1)

5. Possible starting of a flame war or forum warning/ban.

6. Its anonymous

To basically sum it up, why would you...

...take the time out of your day to risk looking like a **** by replying to someones post that it is irrelevant or useless when you can do it in just one click of a -1?