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".