What is the need for going complex join in BMM layer

When ever you build a request in answers BI server first checks in BMM layer to build a query for the request built so you have to have joins right to be able to built query and the main reason is when Complex joins are used in the business layer then they are really acting as placeholders. They allow the OBI Server to decide on which are the best joins define in the Physical Layer to satisfy the request.

Simple understanding is physical layer as it is the first layer in RPD as you import tables you need to establish joins between keys and this is a thumb rule to use primary-foreign key relationships.
Secondly you cant do complex join in physical layer cz you dont mention which key to join in complex join,you just establish a join.  You can use foreign-key join in BMM layer depends on the requirement.

If you dont have joins,then import those tables into physical layer and identify a key column or create a key column in those tables and when you go to properties of the column you find last tab as key ...you need to check or enable so it becomes as a key column.That way you can establish joins.