I have seen people adding spread spectrum voltage controlled oscillator to EMI on the board, How is EMI affected by single spectrum crystal and with spread spectrum crystal.
Spread spectrum improves EMI by spreading out the peak emission. Instead of one very strong spike at a high energy level, you end up with a wider "peak". The power output is the same, at the cost of frequency accuracy.
The benefit to this becomes apparent when you have specific emissions requirements to meet. You may not be permitted to emit more than xmW at a given frequency. Without a spread spectrum clock, you might not come close to meeting this requirement, but if you spread the emission frequency out a little, the power at any specific frequency in that range might fall below the maximum allowed, and now you pass.
The downside to spread spectrum clocking is that your frequency is no longer precise, because it (intentionally) wanders over a wider range of frequencies that are centred around your desired frequency. The average frequency is your desired frequency, but at any given point in time you will somewhere in the spreading range. This can cause trouble if you're trying to communicate with other devices.
Here's an exaggerated picture of the difference between a clock output at a specific frequency vs the same clock output with some spreading. Ignore the "noise level" note, or instead imagine that the dashed line "noise level" is the absolute maximum level that you're allowed to radiate in order to pass emissions testing)
(The picture source is http://www.tapr.org/images/ssfig1.gif.)
My answer is to directly quote wiki
Spread-spectrum clock generation (SSCG) is used in some synchronous digital systems, especially those containing microprocessors, to reduce the spectral density of the electromagnetic interference (EMI) that these systems generate. A synchronous digital system is one that is driven by a clock signal and, because of its periodic nature, has an unavoidably narrow frequency spectrum. In fact, a perfect clock signal would have all its energy concentrated at a single frequency (the desired clock frequency) and its harmonics.
Practical synchronous digital systems radiate electromagnetic energy on a number of narrow bands spread on the clock frequency and its harmonics, resulting in a frequency spectrum that, at certain frequencies, can exceed the regulatory limits for electromagnetic interference (e.g. those of the FCC in the United States, JEITA in Japan and the IEC in Europe). Spread-spectrum clocking avoids this problem by using one of the methods previously described to reduce the peak radiated energy and, therefore, its electromagnetic emissions and so comply with electromagnetic compatibility (EMC) regulations.
It has become a popular technique to gain regulatory approval because it requires only simple equipment modification. It is even more popular in portable electronics devices because of faster clock speeds and increasing integration of high-resolution LCD displays into ever smaller devices. Since these devices are designed to be lightweight and inexpensive, traditional passive, electronic measures to reduce EMI, such as capacitors or metal shielding, are not viable. Active EMI reduction techniques such as spread-spectrum clocking are needed in these cases.
However, spread-spectrum clocking, like other kinds of dynamic frequency change, can also create challenges for designers. Principal among these is clock/data misalignment, or clock skew.
Note that this method does not reduce total radiated energy, and therefore systems are not necessarily less likely to cause interference. Spreading energy over a larger bandwidth effectively reduces electrical and magnetic readings within narrow bandwidths. Typical measuring receivers used by EMC testing laboratories divide the electromagnetic spectrum into frequency bands approximately 120 kHz wide. If the system under test were to radiate all its energy in a narrow bandwidth, it would register a large peak. Distributing this same energy into a larger bandwidth prevents systems from putting enough energy into any one narrowband to exceed the statutory limits.
The usefulness of this method as a means to reduce real-life interference problems is often debated, since it is perceived that spread-spectrum clocking hides rather than resolves higher radiated energy issues by simple exploitation of loopholes in EMC legislation or certification procedures. This situation results in electronic equipment sensitive to narrow bandwidth(s) experiencing much less interference, while those with broadband sensitivity, or even operated at other frequencies (such as a radio receiver tuned to a different station), will experience more interference.
I can't add anything to what is said above. It maybe isn't a total con but it isn't something that is necessarily a useful thing to do other than to get through regulatory tests.