They are excellent for complicated data management tasks. They provide a platform of standardization. There are small, fast, cheap systems that don't have all the bells and whistles of large ones but can often handle the simpler tasks. And each one its time plays many parts Different users of the database often perceive it very differently. Two users of the same database may not even realize it.
Databases can act as the intermediary between multiple agents.
The fundamentals of how database management systems are constructed. The database literature. Conducting database research. Posing a question. Designing a system.