This article didn't make much sense to me. In particular, what was it about the problem that prevents having a "stage" object with a "do_stage" method, which t

This article didn't make much sense to me. In particular, what was it about the problem that prevents having a "stage" object with a "do_stage" method, which takes the input object and returns the output object (or some error code etc.). I feel like the answer has to do with concurrency but the description in the article was to vague to get a more precise idea.