It's a rating and review site. About what?... People (all of us regular folk).
Why?... The question should be, why not?
It doesn't seem fair that someone can just say whatever they want?...
We already do! This site just helps in gathering those opinions.
As tempting as it may be to want to submit dozens of reviews about deceitful, ill mannered, unfaithful, two-faced jerks!... please keep in mind that you also know some good people too. You know what they say, "what goes around comes around."
If 9 out of 10 people say someone is a liar there is a good chance the person is a habitual liar. So for example, you might use this information to decide whether or not you should accept them as your new roommate. See how this information could prove to serve as a credible resource? However, keep in mind that these reviews are just other peoples opinions. You have to decide for yourself what to take into consideration when forming your opinion.