When you can define a cell depending on another cell and can expect that other cell to be run after its dependencies have run, then you can make sure, that a cell will always give the correct result, not a different one, because other cells have already run, which mess up the state. It would allow you to scroll to the end and see the last layer of abstraction, just run that, and implicitly also run all its dependencies (other cells).

In a top to bottom typical notebook, you will have to run the notebook from the top. There is a menu item for that, of course. However, what if you also have some cells in there, which do not have to be run? They will waste time unnecessarily or even worse, will change the result. So you will need to take a look at each cell to make sure you really can run everything sequentially. Either the notebook has to be created to make sure, that running top to bottom always works and gives the same result, or you will have to look at everything. When code segments can define their dependence on other code segments, they can be designed to that it does not matter which one you run, it will always give the intended result.