Basically, it's a distributed code-first workflow manager (as opposed to configuration-centric solutions that use BPML). It's a fork of Cadence, which was the author's project at Uber to create a single workflow manager for their platform.

The use case is basically any kind of temporally-distributed workflow that you might use something like Airflow for, or else otherwise hack together with some combination of event buses, temp tables, and cronjobs. Its main charm is that you define the logic in code rather than templates, so you can intermix business logic directly into the workflow.