It all depends on the type of company where you work as a junior service designer and their training culture. In some companies, being a junior service designer will feel like being a "normal" service designer with a bit more coaching and support. And in other companies, it might feel more like being the assistant.
In my experience, junior service designers help the team by:
Taking on smaller, exploratory projects that the team doesn't have the time to work on. The type on non urgent but interesting work that often gets put on the back burner.
Assisting on bigger projects: for example by taking visual notes during workshops, going through tons of data from user research, building prototypes, etc.
As a junior service designer, you can also be in a shadowing role where you can see how more experienced designers do the client-facing work.