Project types determine if a project should have certain assignments or overrides applied to it.
Checklists
You can configure a checklist template to be associated with a project type. When a new project is created of that project type, the checklist is automatically assigned to it. See Create or modify a checklist template.
Master finding severity overrides
Depending on the project or environment a vulnerability is found in, you might need to assign a different severity than the default to the finding. You can use severity override rules to configure a different severity based on the project type. See Manage severity overrides.