Is it recommended to use Identity as an alternative to primary key?
Identity columns and Primary Keys are two very distinct things.  An Identity column provides an auto-incrementing number.  That's all it does.  The Primary Key (at least in SQL Server) is a unique constraint that guarantees uniqueness and is usually (but not always) the clustered key.  Again in MS SQL Server it is also an index (in some RDBMS they are not as closely tied).  As an index it provides faster lookups etc.  Frequently Identity columns are used as the Primary Key if no good natural key exists, but are not a substitute.