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.
Not sure quite what you are asking. Joins do something different to PKs/FKs - and they are clearly not a substitute for each other. PKs and FKs implement constraints to ensure correctness of the data. Joins are a type of query operation.