Imagine that every Sunday you have the opportunity to design your week schedule.
Now, imagine that you are only allowed to design your schedule by combining pre-built blocks, and that you are given blocks like those in set A.
And now imagine you can choose from blocks like those in set B
Which set of blocks will allow you to build a more flexible schedule? Would the schedule built with blocks from set A be more or less difficult to adapt to change that the schedule built with blocks from set B?
That’s modularity. And that’s why the single responsibility principle matters. And that’s why methods should not be long. And that’s why classes should not be long. And that’s why view controllers that control views, do networking, and contain logic to deal with navigation are bad.