What is the need of surrogate key;why primary key not used as surrogate key Surrogate key is system generated unique number ?
Identity in SQL, Sequence in Oracle, Sequence Generator in Informatica.
For Example : You are extracting Customer Information from OLTP Source and after ETL process, loading customer information in a dimension table (DW). If you take SCD Type 1, Yes you can use Primary Key of Source CustomerID as Primary Key in Dimension Table. But if you would like to preserve history of customer in Dimension table i.e. Type 2. Then you need another unique no apart from CustomerID. There you have to use Surrogate Key.
Another reason : If you have AlphaNumeric as a CustomerID. Then you have to use surrogate key in Dimension Table. It is advisable to have system generated small integer number as a surrogate key in the dimension table. so that indexing and retrieval is much faster.