To be able to easily split a subcatchment into 2 separate subcatchments.
For example when it becomes obvious that the flow from a subcatchment cannot be collected at a pit (node), it becomes necessary to insert an additional pit (node) to capture flows. The subcatchment needs to be easily split (say with a general line) into 2 separate subcatchments with similar data other than the new areas.
Thank you for the suggestion and thanks to all those who voted.
I moved the status of this to "Already Exists" since users can split polygons with a general line object intersecting it by selecting both objects >> Model > Geometry > Crop overlapping selected objects >> Reference the entire selection in both drop targets >> Press "Ok" and the result should be two polygon objects as per the line intersection
There are some additional options here which can be enabled or disabled as part of the process. If this feature does not meet your needs, then happy to consider further enhancements to it, but for a modelling solution hopefully the geometry functions are already fairly advanced.