the DbContext is a smaller API exposing the most commonly used
features of the ObjectContext. In some cases, those features are mirrored in the DbContext
API. In other cases, the Entity Framework team has simplified more complex coding
by providing us with methods like Find or properties like DbSet.Local. But there’s
a big API lurking underneath that you may still need access to. For example, you might
want to work directly with the MetadataWorkspace to write generic code against classes
because that API can read the model more efficiently than reflection. Additionally, the
MetadataWorkspace is able to provide more information about the metadata than you
can discover with reflection, for example, for Key properties. Or you might want to
take advantage of a database-specific function that is exposed through Entity SQL,
which you can’t access from LINQ to Entities.
Or you may already have an application
written using the ObjectContext and you want to leverage the DbContext in future updates
without replacing all of the ObjectContext code.(Reference from Programming DbContext)