Devin Knight has a Q&A on Power BI:
Q: what is the difference between the Query editor and Data Modeler? What can and can’t do in each case ?
To summarize the Query Editor is mainly for Data Extraction actions. So providing source information, applying rules to the incoming data, etc… The Data Modeling areas are focused on creating relationships between tables you’ve important and creating calculations you might need in your report. This of this as the last step to prepare you data for reports.
Check out Devin’s webinar as well. It’s a lot longer than a coffee break, but worth your time.