The database system provides multiple isolation levels for users to choose from, but many users stick with the default level without considering what's optimal for their application. This can lead to performance costs and concurrency bugs, as lower isolation levels expose an application to specific types of anomalies. The SQL standard defines several reduced isolation levels below serializable isolation, including READ UNCOMMITTED, READ COMMITTED, REPEATABLE READ, and Snapshot Isolation, each with its own set of anomalies and potential issues. Understanding the differences between these isolation levels is crucial for preventing concurrency bugs and ensuring correct application behavior. However, the SQL standard's vagueness and ambiguities can lead to semantic differences across implementations, making it challenging for developers to choose the right isolation level for their application.