How to create Views for BW Extractors to use in SLT ?

From multiple Blogs/SDN Q&As I understand that we can not use the BW Extractors directly inside the SLT, But we need to create the Views based on the tables on which BW Extractors were built.

1. Is there any document or Blog which provides idea about step-to-step creation of Views for some BW Extractor ?

2. Why SAP did not come up solution/idea to use BW Extractors directly inside the SLT ?