Sometimes you really, really, REALLY, need to know what it is that you are dealing with. UX projects which start from a blank slate are rare and so there is a frequently a need to analyse and map existing systems.
I call the schematic above a ‘process flow’ rather than a ‘user journey’ as it shows the mechanics (at an interface level) of a system, not necessarily a user’s journey through it.
This is one very small section of a much more complex booking path for an e-commerce system.
Having said that user journeys could quite easily be mapped onto this foundation layer.