Brand/product awareness. Sometimes you don't know what you want. And sometimes you don't know if certain product/solution exists for the problem you are facing. Believe it or not, one of my favorite apps is MyFitnessPal, I downloaded it via an ad because I was looking was health/fitness category product but never heard of MFP (few years ago before they were popular). I thought I will give it a shot and downloaded and solved my problems plus I discovered few more use cases for my health/fitness related interests.
Another example (rather misconception), "Google became famous via word of mouth". No, Google had prominent ad placement (aka powered by Google which was part of their contract) as a part of their deal with providing search on AOL/Yahoo portals. When non-tech savvy users searched on these portals they were exposed to Google and were exposed to much more superior search engine than that was available.