If a phase is added to a topic, it is visible to users as a journey. The individual phases can then contain decks or tests. Phases can be unlocked by progressing through the previous phase.
Concrete use cases for journeys are:
Temporal phases, e.g. in onboarding. "Your first week of work, your second week of work, ..."
Presentation of knowledge levels. "Level 1: Basics, Level 2: For advanced users, Level 3: Expert knowledge, ..."
Illustration of longer courses. "Preparation, information on seminars, summaries, ..."
You can find out how to create a journey in the article How do I create a learning journey?