Report Templates are not necessary to begin a report in PlexTrac, but they perform two very useful functions:
They allow you to pre-populate your report with pre-filled sections of a report, such as Introduction, Methodology, Threat Model, etc.
They allow you to link a custom export template, so that your final .docx is exported into a document that is branded for your business in a manner that suits your reporting methodology
You can choose one or both of these options when creating your Report Template; you do not have to do both. Additionally, you can change the custom sections or the export template at any time - even after you have completed writing your report.