Why do we need SQL queries
The answer is quite simple. As a Bizagi BPM Consultant/Developer you must have a way to better understand how things are managed at a database level. This can be achieved by running some custom SQL queries.
I mainly use these scripts to instantly get access to some information that otherwise is quite impossible to get from Bizagi. Also, it provides an amazing way to debug your process, check for performances issues, bottlenecks in your workflows, etc.