Please help me to understand why we need to create extra module named country margin report when we can take same data by filtering data from already created module named Margin calculation.
You're right, from the point of view of economy of sizing and not to bog the information, it makes sense to use one module and filters. But this is a training course and to create skill, demonstrate D.I.S.C.O. and not to complicate the process (Through the setting of filters) is used this approach with separate modules. Including it shows how the linking of modules with different levels of detail works.
In real projects, it's really better to use one module and it's different views.