>> Also, I would think in-app purchase pricing would be relatively inexpensive. I guess I don't see who they're targeting... the target company would need substantially-priced in-app purchases to justify the cost of even traditional advertising.

Not sure what you mean. Why does in-app purchase pricing have to be inexpensive? Why does it matter if it does? Traditional advertising is substantially more expensive than mobile advertising.

The reason a company would be interested in this is because someone who has your app installed (say Uber, to take one of the apps that enables transactions on my phone as an example) is one of the highest value customers you can reach in terms of likelihood to make a purchase (they installed your app, so it's akin to someone entering your store).