The application workload is distributed along many computers. Each one specialized in the job that they are doing.
Your PC, tablet, TV with web browser is made to interact with you. They are optimized to provide an interface between the application and humans. The system gives priority to anything related to data display and user input.
The Web/Application server is optimized to process data, produce fast calculations, apply business rules. This is a machine with powerfull processors, small but fast disks. This is the processing and control unit of the application.
The data storage. The unit responsible for storing, relating and retrieving the necessary information as fast as possible. This machine, with large discs, and fast input/output is tuned to process all the data of the application. Retrieving the data is only part of the functions, the other parte is keeping relations between all data.