procedure vs function

1) Wanted to get the some return value. He can acieve it with both function and procedure .With function if he want to return a single value he can use return statement . If he want to return multiple values he can achieve it with inout parameter.Similarily he can get return value with inout parameter from procedure(not with return statement)

But it does not make any difference to developer as long as he is able to achieve its intentention either with return statement or inout parameter.

so here also both can replace each other.

2) He can use DML in both Function and procedure. So here also he can use either of these to change the state of databse.

So i dont get any concrete reasoning which one to use where as both can replace each other in some.

The only reasonable reason i found up to some extent is that Functions can be called from SQL, procedure cannot

Could somebody explain which one to use when and why?

You already found the main difference. You create a function if you want to use it in SQL. You create a procedure, when you want to use it only in PL/SQL.

There are main two different:

1:Use Procedure to take some action. But use function to return some value. 
2:You can call function from sql query but Procedure can't. 
3:Best practice to use Procedure then function if possible. 

A procedure and a function have the same structure, except that:

A function heading must include a RETURN clause that specifies the data type of the return value. A procedure heading cannot have a RETURN clause.

A function must have at least one RETURN statement in its executable part. In a procedure, the RETURN statement is optional. For details, see RETURN Statement.

For more information refer to: