This will probably be moved to [meta](https://meta.superuser.com/)

In short, users who are active on other sites in the stack exchange network [gets an association bonus of 100 reputation points](https://stackoverflow.blog/2009/07/08/cross-site-account-associations/) on this site. The requirement is to have above 200 reputation points on an SE site.

The reason is that such users probably know the format, and some of the restrictions for new users, such as commenting, doesn't make sense for users that knows the format.