According to the American Economic Review, 56 percent of American Adults had a smartphone in 2013 and the growth is still going up (American Economic Review).
Think of how your application is going to scale on Mobile Phones. Google’s Search algorithms now take into consideration the responsiveness of a Web Application on different Mobile platforms and then promote those that meet the standard (Search | Google Developers).
Different clients (Mobile, web, and Desktop) would need different requirements to better serve their needs, for this reason, it is important to have one central or distributed Database that grows horizontally in order to leverage Sharding (copying of data or distributing data to different nodes on a network).
One way to make sure that your application provides valuable data across multiple devices is by using a Web Service that provides Methods/Functions that can be called in form of an API Endpoint. This is done so that different clients like Mobile Apps, Desktop applications, and Web Applications can access the same End-Point and use the same Database.
Planning your Application Design or architecture becomes the laying foundation of most problems you will encounter in the future when the application scale to a Million concurrency.