Just like how, in writing a story, a writer must also toil over each sentence, and should this be an emdash or a comma? and should I break the paragraph here or there? All this minutia is just as important to the final product as grand ideas and architecture are.
If you don't care about those little details, then fine. But you sacrifice some authorship of the program when you outsource those things to an agent. (And I would say, you sacrifice some quality as well).