All menus have to be suited to one business application. Each user must see all menus linked to their job. They should perform their workload in one menu only. This is the goal of all the applications. The user should be able to navigate through all his workflow from a single menu to make his work easier, faster and user friendly.
Menus are defined in a certain way to make the work of users easier . So, users can see all the menus they need in a logical order, to facilitate navigation and use. A salesman doesn't need to see the invoices before he has even made the quotation.
Main menus are also organized to facilitate the work of a manager. Indeed, in an SME, a manager needs access to many applications: Sales, Projects, Human Resources, Marketing and Accounting. When he opens his instance of OpenERP, he has to see main menus in a logical order.