Rating determines two things - whether you can rate others and whether your posts are visible.
If your posts are rated down they drop out of "normal" visibility. Up the top where it says "Forum preferences" one is "Default Filter". This can be changed to show all posts or only ones rated "Good" or better etc. Since most people neve rbother to change it a rated down post can drop out of the thread.
As to why people do it to perfectly innocuous posts I have no idea. It is there to let the forum police itself and get rid of obnoxious posts.
I think below a general rating of 3.00 you cannot rate others.
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Wherever I go - there I am.