How does a Content Type in the Document library usefull? If new Content Type Template is added how to apply the existing Columns to it?
There are many articles explaining this concept, however one of the main benefits of content types are defining specific columns / fields that are associated with a type of content. For example if you defined a "Quaterly Report" content type, you could associate the columns "Year" and "Quater" with all quaterly reports.
Content types are first defined centrally in the Site Content Type Gallery for a site. Content types that are defined at the site level are called site content types. Site content types are available for use in any subsites of the site for which they have been defined. For example, if a site content type is defined in the Site Content Type Gallery for the top-level site in a site collection, it is available for use in lists and libraries in all of the sites in that site collection. Site content types can be added individually to lists or libraries and customized for use in those lists or libraries. When an instance of a site content type is added to a list or library, it is called a list content type. List content types are children of the site content types from which they are created.
When you add a content type to a list or library, you make it possible for that list or library to contain items of that type. The New command in that list or library lets users create new items of that type. One of the key advantages of content types for lists and libraries is that they make it possible for a single list or library to contain multiple item types or document types, each of which may have unique metadata, policies, or behaviors.
To add an existing column to a content type:
Go to site settings > Site content types and locate your content type you wish to modify.
Then towards the bottom of that screen you will see "Add from existing site columns".