If it is across a switch, I am guessing that the switch is controlling an inductive load. In which case, it is intended to absorb the turn-off transient and spare the switch contacts from arcing.
Spread spectrum clocks are definitely helpful for passing EM emissions tests - without, as has been pointed out, actually reducing the total noise energy.
Whether the "smearing out" of the noise spectrum is actually advantageous (other than in passing the test) depends on the nature of the...
