What is the purpose of Master Display in Views module of Drupal 7? Why is it not present in every Drupal 7 installation?
To always see the Master Display you need to set the 'Always show the master display' setting at 

admin/structure/views/settings


The Master Display is a convenience and time saver when you have a set of related displays (block, page, feed etc.). You create the master view and then all of the other displays are 'clones' of the master that you then customize individually. Also, when you revert a setting in an overridden display it will revert to that of the master. I also think that the master display is fully saved in the database, and for all the other displays on the same view only the overridden parts are saved, hence less storage.     

The Master Display has no associated menu and is therefore good when you need a view but do not want to unnecessarily create a menu entry (ie page) or block or other views resource. It is typically used in this fashion when embedding a view in code, or by other modules that take a view as a parameter, ie Answers, and Views Field View.