A lot of the focus here is on one product businesses, like Coca Cola. These type of companies can both brand and sell in the same ad. For anyone in a multiproduct business, both threads need to be approached differently. In fact a response to a "buy now" can be converted into an "image response" and vice versa.
Heres a test I did a few years ago. I ran two ads in a respected specialist journal for six months.
Ad 1 was a generic "We do this, stock that, know about blah and blahblah."
Ad 2 was a technical product that would have a small proportion of the specialised readership salivating to own it.

The "positioning" or image building ad 1 generated a lot of enquiries and some new business. A spin off effect was a kind of subliminal effect of "I have heard of these guys before" kind of feeling. Very hard to measure ROI, gut feeling is that it was worthwhile.

Ad 2 generated a litle less response than we thought, just broke even. It could have been tweaked, but had very little residual pulling power and targeted only a small part of our potential audience.

If my business had been solely dependant on the product in ad 2 I would have run "buy now" ads that strongly branded my company as the prime source for this type of product. Exactly as Coca Cola proclaims itself to be "The Real Thing".

At the supermarket level, cheap Coca Cola is just one of the come-on offers a store may make. They may even promote "off brand" cola in the same flyer. Whatever gets you in the door. Protecting and building the brand is the manufacturers job, moving the stuff out the doors is the retailers job.