One of the strengths of the IfcOpenShell library is the vast amount of shape analysis and healing tools that Open Cascade provides. This allows for transforming, validating, generating and analysing geometry in IFC files, for example, to compute missing IfcProperty values, validate geometrical constraints, implement clash- and interference checking and the generation of new geometry.
pythonOCC exposes the rich set of functionality that Open Cascade provides to a Python environment, which allows for intuitive rapid prototyping. Since IfcOpenShell also provides a Python wrapper, the two of them can be easility tied together to create a development environment for working with geometry in IFC files in an interactive, quick, but also, very powerful way.
To demonstrate the possibilities, or rather, the first baby steps towards a more interesting and meaningful demonstration, I put together a quick example here: http://ifcopenshell.org/pythonOCC/example1/. The set of dependencies to get the system running is long, but it is worth it!