The expansion/gating algorithm often needs to make very quick changes to the audio when opening or closing. These sudden changes can introduce aliasing, which causes distortion and generally reduces the quality of the audio signal.
Oversampling is a way to reduce that aliasing by running the internal process at a sample rate that is two or four times higher than the host's sample rate. When should you use oversampling? You need it more when when you're using very short attack or release times, especially in combination with a high ratio and range.
Of course, in return for a reduction of possible aliasing/distortion, the plug-in will use more CPU power when using oversampling. In addition, oversampling introduces a small latency, in addition to any look-ahead latency.
Next: Mid/Side processing