Scenario A: Imagine a list of blog articles, all controlled by a CMS collection. However, within that list (let's say the 3rd item from the top) the client wants to insert a "sponsored ad" type element, perhaps promoting something or even just a custom code DIV that can house an external ad.
Scenario B: Imagine an agency website that has a careers page. In that page there is a grid (let's say 3 columns across) of all the current job positions available, and each one links through to the job details template page. At the very end of the list, there is a static "if you don't see a position for you, contact us anyway" type block, but the client wants it to sit *within* the grid list, not as a seperate section underneath. For instance, in a 3-column grid, if the last job position item was sitting at #1 or #2 in the current row, then the static block should sit in position #3, so it looks like it is part of the list, while being static.