Comment by williamdclt
1 day ago
You're not missing something, but you're assuming that it's easy to know ahead of time where the module boundaries should be and what the interfaces should look like. This is very far from easy, if possible at all (eg google "abstraction boundaries are optimization boundaries").
Also, most of these interfaces you'll likely never need. It's a cost of initial development, and the indirection is a cost on maintainability of your code. It's probably (although not certainly) cheaper to refactor to introduce interfaces as needed, rather than always anticipate a need that might never come.
I think it is more intuitive if we think of side-effects. By specifying the interface you are explicitly defining inputs and outputs. If you want to add this later, it can be very difficult to make sure you can find all the side-effects. The whole point of the interface is to explicitly limit those side-effects and extra inputs outputs to happen, so it makes sense to define in advance.