Having a default project can greatly speed up the issue creation. Once you open the create issue dialog all fields are immediately rendered. There are multiple ways to set up a default project.
Last used project
You can enable a setting to remember the last used project and automatically select it again next time.
Based on subject
The add-in checks for a project key in the summary in format key: and [key]
Example for Project key DEMO
Key: ---> Subject: DEMO: new features requested
[KEY] --->Subject: [DEMO] new features requested
This can be handy, when working with multiple projects and external collaborators.
Admin controlled DEPRECATED
In previous versions the admin could control a default project for all users centrally. This has been removed in favor of the methods above.
Slightly related, even if you work with multiple projects and a single default project does not work for you, the add-in will suggest your recent projects.