What's its purpose of this? I think I am the only one I rate threads...

As you said, to rate the threads...but no one really pays attention to it, so it's not really of much use.

The typical person won't even notice it anyway, so there's not much use for it. And while I could make it more noticeable, people might rate threads unfairly, for example someone might rate others threads lower for no reason. I doubt that would happen, but it's possible and I rather avoid such problems. Plus people might ignore such threads due to the low rating, when in reality it might actually be a good thread.

I didn't know you could rate anywhere.

now it has a rating of -123123131231 hahahaha lol

It's found below the quick reply box.

I posted a picture of where it's located: