One thing I've always thought about ads is the subconscious impact that they have of building brand recognition and awareness. Especially for something which would typically be purchased infrequently or irregularly.

I might be at a grocery store and I need to purchase something like dishwashing liquid. If I'm staring at two brands on the shelf priced identically (or near enough) I'm more likely to purchase the brand I recognize over a brand I've never heard of. How do I recognize the brand - from advertising.

That is not something you can measure with tracking metrics embedded into a banner ad. I'm sure there must be studies on impact this has.

The other thing I suspect online advertising works very well for is service based businesses, the kinds of things you used to look up in a phonebook. The last time I needed to find a plumber I typed my suburb name + plumber into search engine I imagine a lot of people do similar.