Suppressing "travelling" EMI spikes
Normally "traveling" peaks do not contribute meaningfully to quasi-peak or average measurements, which are (usually) what counts when it comes to EMC regulatory standards.

Frequency dithering can induce traveling peaks - this is a standard EMC "cheat" for power supplies and is a feature of some PWM controllers to specifically beat the QP and average emissions limits.

A QP detector is sort of like a peak detector followed by a lossy integrator. Spurious peaks (ones that aren't always there) tend to get filtered out. Average detectors are even less sensitive to spurious peaks. These travelling peaks only show up in the QP and average measurement frequency windows for short periods of time, hence their minimal impact.