Help/contribute to the architecture of new applications and databases so the app dev and data engineering teams design solutions that will perform well from a data perspective. They want to avoid mistakes that would cause performance issues down the road.
Help optimize and tune existing solutions.
Some of their existing applications and data processing routines have performance issues which might require altering the approach or refactoring the SQL code.
The SQL Developer would be responsible for suggesting ways to improve performance and implementing those improvements.