Labels are frequently used to organize stories across your projects. Labels are useful to split your project into different modules, different teams, different concerns.
Insight enables you to filter most charts and widgets by by labels. It is simple way narrow down your information to a specific piece of the project and eliminate the noise.
Insight enables you to track labels and epics across multiple projects. It is as simple as filtering by multiple projects and choosing the shared labels from the widget configuration.
For example, you could track the progress of the epic "Back-end" across your 2 main projects (Project 1 and Project 2) for the current iteration.