Why would you choose a database system instead of simply storing the data in Operating system files?

storage devices such as disks or tapes. This data can be maintained as a collection of operating system files,or stored in a DBMS (database management system).
A database system is used for storing data of any kind. A text file is not stored data, so you wouldn't use a database for THAT. But a dictionary of words used in the text file, along with their definitions, could use a database. 
Even pics can be stored in a database, and even a list of other files can be stored in a database. 

A database is an integrated collection of data, usually so large that it has to be stored on secondary storage devices such as disks or tapes. This data can be maintained as a collection of operating system files, or stored in a DBMS (database management system).

The advantages of using a DBMS are:

• Data independence and efficient access. Database application programs are independent of the details of data representation and storage. The conceptual and external schemas provide independence from physical storage decisions and logical design decisions respectively. In addition, a DBMS provides efficient storage and retrieval mechanisms, including support for very large files, index structures and query optimization. 
• Reduced application development time. Since the DBMS provides several important functions required by applications, such as concurrency control and crash recovery, high level query facilities, etc., only application-specific code needs to be written. Even this is facilitated by suites of application development tools available from vendors for many database management systems. 
• Data integrity and security. The view mechanism and the authorization facilities of a DBMS provide a powerful access control mechanism. Further, updates to the data that violate the semantics of the data can be detected and rejected by the DBMS if users specify the appropriate integrity constraints. 
• Data administration. By providing a common umbrella for a large collection of data that is shared by several users, a DBMS facilitates maintenance and data administration tasks. A good DBA can effectively shield end-users from the chores of fine-tuning the data representation, periodic back-ups etc. 1 2 Chapter 1 
• Concurrent access and crash recovery. A DBMS supports the notion of a transaction, which is conceptually a single user’s sequential program. Users can write transactions as if their programs were running in isolation against the database. The DBMS executes the actions of transactions in an interleaved fashion to obtain good performance, but schedules them in such a way as to ensure that conflicting operations are not permitted to proceed concurrently. 
• Further, the DBMS maintains a continuous log of the changes to the data, and if there is a system crash, it can restore the database to a transaction-consistent state. That is, the actions of incomplete transactions are undone, so that the database state reflects only the actions of completed transactions. Thus, if each complete transaction, executing alone, maintains the consistency criteria, then the database state after recovery from a crash is consistent. If these advantages are not important for the application at hand, using a collection of files may be a better solution because of the increased cost and overhead of purchasing and maintaining a DBMS.

An Operating System file is a file the system need to run in order to function and is unrelated to the information that is stored in a database. The database is also for searching, record keeping, record retention and analytical purposes and relates to events of the company as opposed to making the computer function.