The biggest advantage of using a mobile app to promote your business is that you can be "ProActive" vs "Reactive." Mobile-friendly websites are at a disadvantage, because the customers may not know you have something new to offer. There is no way to alert them unless they're on an email list. Even then, this is a complicated process for the user. They have to open their email program, then go to your website on their phone and find the info. There are just too many steps for people to have to jump through to get the info. It's not a seamless process like accessing the info using a mobile app.
Mobile apps have access to features like "push notifications" that can prompt the user to not only know there's new content to check out, but also where to find it.