Modularisation
DBT allows users to break down complex data transformation into smaller and reusable components. These are called models. Each model performs a specific transformation on the data and therefore, can be reused in other parts of the worklow. You would simply reference this model in another workflow and it saves the hassle of looking back over a huge amount of SQL trying to figure out what it does or where you went wrong.
SQL: regarding complex queries, SQL will often result in large scripts. CTEs do help to break up large pieces of code however this could still be confusing.
Data Testing
DBT has the ability to define tests on your data models. You can write tests to ensure there are no duplicates in your data as well as any nulls. This can also ensure data quality.
SQL: doesn't have built-in mechanisms for data validation or testing. SQL requires users to manually write queries to check the data integrity.
Documentation
DBT has a docs feature that generates a web page that shows models and their relationships. This helps with other users understanding their work or passing this on between colleagues.
SQL: queries don't include automatic documentation, users would have to manually leave comments between the code. This can make SQL quite hard to digest without these comments.
Model Management
In DBT, models can depend on other models. DBT automatically handles the order in which models should be run based on these dependencies. This automatic management makes it easier to work with complex data pipelines as the user doesn't need to worry about the execution order.
SQL: typically requires users to manage dependencies manually, which can become confusing as number of transformations grow.
SQL Optimisation
DBT complies your SQL queries to optimise them for execution in your target database. It can also handle materialisations (e.g. tables, views etc.) to make your queries more efficient.
SQL: the user is responsible for optimising their queries and also managing the creation of tables and views.
Conclusion
DBT can add a layer of ease for complex transformations. While SQL is essential for querying and transforming data, DBT can provide better documentation and a more visual aspect than working with SQL alone.