The most common use case of multi-axis manufacturing systems is setup reduction - being able to perform more than one, non-multi-axis operation within a single setup
Certain geometries necessitate the use of simultaneous, multi-axis toolpathing. The most ubiquitous examples of parts requiring simultaneous multi-axis toolpathing are impellors and turbines, which cannot be manufactured using simple machines/methods.
The ability to reorient cutting tools for optimal tool/cutter engagement is an oft-overlooked aspect of multi-axis manufacturing, and can both significantly increase tool life but also decrease cycle time for many products