Directly attached storage (DAS), as the name implies, is plugged into and controlled by a server. It is dedicated to the server and originally held the operating system and all applications & data that ran on the server.
As technology evolved, storage has moved to shared resource pools configured in Storage Area Networks (SAN) and Network Attached Storage (NAS). This allows the storage to grow as needed without forcing changes to the server configuration, reducing complexity and cost associated with storage capacity changes.
Creating a tiered storage architecture is also common because modern data-hungry applications such as analytics or data mining can be matched to the appropriate storage platform based on their value to the business. For example, a Wall Street trading analytic application may need all-flash storage ($$$$) for performance where the ROI justifies the expense. On the other hand, mining the last two decades of sales data for trends to determine appropriate inventory volume for the next year may be housed on disk storage ($$) due the non-urgent nature of the results.