I noticed that nginx users tend to create a separate group and user for nginx. Why is this?

Further, the default path for the HTML public directory is /usr/local/nginx/html, but people use the home directory instead (e.g. /home/newuser/mysite.com/public). The same goes for log files and maybe some others. I suppose the reason is the same as for my previous question?

I'm basing this off a script called "centmin", mainly, but I've seen other resources follow the same trend.