It was first created in "serious" forums, to see how much the other person is :
-smart, interesting, a good guy, knows how to buy other opinions, helps newcomers, etc...
OR
-troll, useless spammer, someone who loves being the baddy even though he's just a kitty, etc...
It reflected how a person was "helping" the forum, and his reputation.
But here... a lot of people are just putting -1 on everyone, for the lulz, and some are even asking for it to be negative. So, it doesn't help at all, and is here, just for fun.