Q: I have always wondered what exactly to say when, during the interview, you are asked something like "Why do you want to work for xyz?"
This is a "bow and scrape" question, and I am sorry that you have to be subjected to this sort of thing. Some interviewers want you to grovel a bit and prove that you are worthy. Nonetheless, you still have to come up with a solid response.
The first thing to recognize is that this is an aggressive question, and aggressive questions should be answered with aggressive answers. Since this is a rather stupid aggressive question, you would be best off tactfully redirecting your response by answering a slightly broader, and better, question. What your interviewer really should be asking is why you are interested in a particular field, not a particular firm. Answering this question will get you where you need to go, but with more useful, honest content:
"From my past work experience and independent research, I realized that my skills are a strong fit for advertising and I am confident it is the right career direction for me. Being an assistant brand manager would be a great way for me to enter this business and I would be very excited to start my career at Acme, a recognized leader in this field."