[SOLVED] series cap?

[hunas2127]

I often see capacitors connected in series in circuits. What is the reason for this?
Is it for isolation or to achieve a smaller capacitance?
I'm curious if this structure has any specific advantages.

 

[vfone]

If the capacitor is used as a coupling capacitor, if place two capacitors in series, and if the capacitors have two different SRF, then the bandwidth of the structure increases.
If place two identical capacitors in series, theoretically the breaking voltage doubles. But in reality this is not happening, because the caps are not perfect the same, and parallel high values resistors are necessary. This is not always possible.
The capacitance of two series capacitors is smaller than the value of the smallest capacitor.