I recently took a look at the list of registered users of my site and found that there truely are some people visiting this place.

However I do not really know why they registered (except for the very few who added comments to some posts)?

Registration does not bring any advantages/services/etc. with it. One of the users even uploaded a small picture to her user profile.
Currently the only way of interaction is adding comments to my posts and this is available for anonymous visitors, too. This is quite a restriction so I considered adding a forum (in Drupal this is just a few clicks), where registered users can add questions/topics not related to any of my posts.
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