I am writing some webservices returning JSON data, which have lots of users.
What are the benefits of using Nginx in front my server compared to just using the go http server?
It depends.
Out of the box, putting nginx in front as a reverse proxy is going to give you:
The Go HTTP server is very good, but you will need to reinvent the wheel to do some of these things (which is fine: it's not meant to be everything to everyone).
I've always found it easier to put nginx in front—which is what it is good at—and let it do the "web server" stuff. My Go application does the application stuff, and only the bare minimum of headers/etc. that it needs to. Don't look at putting nginx in front as a "bad" thing.