They are nothing more than a visual cue of others opinions. Imagine you're standing around in a group, and someone says something. Some in the group will simply "raise their eyebrows", or frown, or laugh or .. they will make a visual indication, rather than say something. Sometimes it suits. Forum readers can have their say without having to post. They also serve another excellent purpose. Once a post receives a certain number of thumbs down, it pops up on our screens as needing attention. Sometimes "off" posts need context and can't be identified by obscene words, etc. The thumbs system helps your hosts keep the forums friendly and a welcome place to visit.