IIRC what it does is add some (tiny amount of) jitter to the clock signal; what this does is to "flatten" the spectrum of the emitted signal slightly (because instead of having a peak at 1GHz, let's say, it's going to be spread around, but centered on that frequency)
The reason the feature exists is so the equipment can pass EMC (electromagnetic compatibility) tests such as FCC Part 15 for emissions. By spreading the radiated energy further out across the frequency domain, the peak amplitude is reduced and you stay under the limit. Of course this raises the noise floor for everyone, but my sheep ought to be able to graze in the common.
