There are lots of answers to that, but the basic one is that we've allowed too many cars into a space which can't handle them. A congestion fee changes that, by actively discouraging them.

If people are driving in circles in the already-super-congested Manhattan traffic instead of going somewhere and parking, then yes, they shouldn't be doing that.

I don't know what gives you the idea that poor people can afford to drive in New York City in the first place. Do you have any idea how expensive parking is?