Question: If i use Natural joins than what is the use of primary key and foreign key?

Answer: 
Primary keys and foreign keys are types of database **constraint**: their purpose is to ensure the integrity of the data, not to facilitate joins.

**Primary Key**: there must not be more than one row with the same value for the primary key column(s).

**Foreign Key**: every value in the foreign key column(s) of each row must correspond to the primary (or unique) key of a row in the referenced/parent table.
Natural joins are usually a bad idea. You can accidentally join to the wrong field and then your results are all wrong but may appear to be right. Even if they work at first, changes to the database at a later time could drastically alter your query results and be hard to figure out. I would under no circumstances use a natural join. 

The PKs and Fks are there to enforce data integrity, it would be an extremely bad idea to remove them whether you use natural joins or not. Databases without proper PKS and FKs almost always end up containing bad data.