Feed-forward capacitor in SMPS feedback
At start-up, or during any high-frequency transient, the capacitor will effectively be a short-circuit. This means the feedback voltage divider will look like this:







simulate this circuit &ndash; Schematic created using CircuitLab

Because of this, the feedback pin voltage will be a lot higher (with respect to VREF) than it would be without the C3 and R4. This higher voltage means there will be a higher error, which means the control system regulating the output voltage will ramp harder towards the correct voltage, to achieve stabilisation. As it tends towards this, the capacitor will slowly become higher and higher impedance, and eventually the C3/R4 combination will be an open-circuit, and essentially not even there at steady-state.

The only draw back is the potential for instability if you choose incorrect values. Every component at every part inside a SMPS circuit affects the transfer function, and therefore can effect how the device performs.

This method of having a faster-response to transient/start-up can be used on anything that has some sort of feedback mechanism with a voltage divider. 

The reason you don't see it everywhere? Well, it's not really needed. The system will stabilise eventually with or without it. It's just a nice feature to improve high-speed performance.