Why and when we use function in SQL?

Ok .and other thinks

we can't use single select in stored procedure because stored procedures execute for all but function we use like select name, address, dbo.functionname(gender), contact from table.

Hi puneet Thanks for the update. But we can use select stmt from the stored procedure so why we used function.its my interview qus so what and can I gave to next interviewer?

Before SQL 2000, User Defined Functions (UDFs), were not available. Stored Procedures were often used in their place. One of the advantages of User Defined Functions over Stored Procedures, is the fact that a UDF can be used in a Select, Where, or Case statement. They also can be used to create joins. In addition, User Defined Functions are simpler to invoke than Stored Procedures from inside another SQL statement