A SQL database serves as a structured data management system, allowing users to efficiently store, retrieve, and manipulate data using structured query language. Its purpose is to maintain data integrity and facilitate complex queries, enabling organizations to manage large volumes of information coherently. SQL databases are crucial for applications that require transaction management and data relationships, such as e-commerce sites and business applications. They offer scalability and support for ACID properties (Atomicity, Consistency, Isolation, Durability), ensuring reliable data transactions.