The goal in any marketing / advertising campaign is to get customer feedback. The quicker you obtain that feedback, the quicker you can fine tune your campaigns to deliver better results! Mobile apps allow opportunities for customers to interact with the business, letting them know what they like and dislike about their products or services. The quicker you can respond to customer feedback, the less it will cost you in advertising dollars!
Mobile-friendly are not often used in this capacity. Most people use mobile-friendly sites as a way to get information. Rarely, do customers think about using mobile-friendly sites as a way to provide customer feedback. Interaction is usually done using a mobile app of some kind.