It's not just about 'fixing errors'.
Fun fact: if there is an error in the included workflow, JMP will stop while running it and the workflow will stay open. This is similar to the 'open' functionality, which I miss, but you need an error in the included workflow to access this feature.
Besides debuging, it's also about understanding an existing master workflow and taking a look at an included workflow to see what's going on.
Some users may be familiar with LabVIEW and its sub-VIs. Workflow Builder, with its ability to include other workflows, is like a draft version of this concept. Error handling is already included.
The ability to open an included workflow will give the users even more of the work-with-subVIs-feeling.
Workflow Builder - open included workflow
The ultimate step towards subVIs:
a framework for input and output variables.