WSGI: what's the purpose of start_response function

Could you supply a real-life example of WSGI start_response function? (Web-server provides that function to wsgi application)

I can't understand the purpose of introducing the start_response.

(I've read like 10 identical texts about the WSGI standard. They all say "WSGI standard is..." None of them says "WSGI is designed this way in order to..." :()