A pick behind the screen of developer work day.

Let say you are the customer  and you have a little great idea of a new feature. Nice :-)
You go to a recommended product manager and let him know about it, together you define feature specifications.
Product manager goes to his developer and translate your idea to  him, then, 
together they analyze your feature specifications and define requirements. 
Simple words- what would it take to bring it alive.
They will give you pricing and when it should be done.
You'll decide if you want to give your project a  green light.
    

So  developers have the well defined requirements and you have a  release time and understanding  of costs and benefits.
NOW, you, the product manager and the developer are in the know zone- 
Time, Specifications, Requirements, Costs and Benefits for your new idea. 
    

Green light:
If you decided to give it a green light, you're work is done, Now its developer time to start working. 
    
    He creates of high level design and flow chart.
    He run designed system on scenarios virtually.
    He defines working Iterations.
    He develops feature on local machine.
    He move new feature to  test environment.
    He ask from QA for tests.
    He move new feature to  staging(pre production) environment.
    He ask from QA for tests.
    He ask product manager for review
    He move your feature to your production environment.
    He ask from QA for tests.
    He tell to product manager :"I'm done!"

Now product manager tell to you : "We are done!"
You are happy with  "a little ;-)" change  in behavior of  "a very small ;-)" button.