Let me give you an example.
If I have to regulate an output voltage, Vout at 5Vdc, from an input voltage Vin at 8Vdc, the common rule of thumb to use capacitor of suitable rating will be:
5Vdc * 120% = 6Vdc, so I can use 6.3V or 10V that are commonly available.
8Vdc * 120% = 9.6Vdc, so I can use 10V or 16V that are commonly available.
This rating for capacitor is intended for allowance for voltage spike, aging (cap will not always perform as specifiec forever), derating factor (due to operating in higher temperature, cap may not perform constantly as well) and drift (tolerance due to manufacturing process, aging and temperature).