Projects are useful as a way to organize your work into separate spaces, or even do versioning or forking for all your projects.
When you first login into Tenderly and start using it, you will be prompted to create you first projects. After that, you can select your project(s) from the dropdown or create new ones:
If you are a part of an Organization you will be able to access all projects that belong to it, and they will be visible in the dropdown. Alternatively you can click on View All.