If I earn 20k rep I will be a trusted user, and for 25k rep I can get access to site analytics.
What about impact/people reached? If there's no benefit, why is it counted?
There's no benefit. It's just for fun.
A lot of people post on Stack Exchange to help people with their questions and answers. Seeing a concrete of number of people "helped" by their content is a good way to quantify that and give people an acknowledgement of their contribution and motivation to contribute more.