Users cannot see it by default. In order to see it on pages where it has been placed, users must click an option in Special:Preferences.
In general, categories should not be hidden. However, in the case of categories which are created solely for use with advanced coding functions like DPL, or when they have been created in order to facilitate achievements, it can be useful to hide them.
Please do not remove this message from this page without discussion in the forums.