A Master Calendar is a table in your data model that various other tables can link to based on a given date (SaleDate, InputDate, BuyDate...). The main advantage of a MasterCalendar is that you can have various flags associated with the dates. Flags such as InCurrentQuarter, Year-To-Date, Previous-Year-To-Date, Month, WeekStart and so on. Because these values generally will have either a 1 or NULL as their value, it makes Set Analysis SUPER EASY and more importantly, super readable.
The autocalendar intends to simplify the date-handling for the beginner (whereby these kind of wizards rather prevent a deeper understanding of the matter) but I'm not sure that in advanced scenarios it has more benefits against a classical master-calendar and in a professional environment I doubt that it is sensible - so it depends on your use case if it's a useful feature or not.
More to this topic could you find here: How to use - Master-Calendar and Date-Values and here: http://www.naturalsynergies.com/q-tip-16-whats-special-about-those-derived-fields/