Differential Pair Mode Conversion
Suppose we have a differential transmission system.
Then if there is a common mode signal present in the system that gets converted to single-mode, it will cause interference with our desired differential signal.
If the differential signal is converted to common mode, it is likely to cause radiated emissions.
Therefore both conversions are undesirable, and it's common to measure or simulate them to ensure they are within acceptable limits.
In other cases you might be designing a structure or device actually intended to convert between differential and common-mode signals (i.e. a balun) and you would want to measure or simulate the performance to demonstrate as near to complete conversion as possible.