My book sells everywhere!

No a book doesn't sell because it's out there on the shelves in the bookshops or it's been written about and publicised on the internet. It is very important to identify the readership and then target the sales of the book  accordingly.
One has to identify the connect between the offerings of the book and the expectations of the potential readers. It helps in promoting the book accurately and thus generating sales and revenue.

Marketing costs money!

Thus it needs to be smart and should create a substantial Return On Investment.
Generally marketing promotions begin with sending questionnaires to the authors. It helps in defining the readership arena for the proposed title. It also helps in building up the resources in terms of contacts for promotion of the book.
After all its the age of customisation and we need to have tailor-made plans of marketing for every title.