The calendar administrator can disable any calendar view. This can be helpful if some views are not practically useful. By removing the views that aren't relevant for day-to-day calendar use, you encourage users to stick with the best calendar views for your needs.
When a calendar view is disabled, it no longer appears as an option in the view picker.
For example, here is a view picker with only 6 calendar views enabled:
Even if a calendar view is disabled, it is still usable via link parameters.
A disabled calendar view can still be used for an embedded calendar.
Any user can also use link parameters to use calendar views with different configuration than what the admin has set. For example, the admin may have set the Multi-week view to show 3 weeks. A user can append view=mw6 to see their calendar in Multi-week with 6 weeks.