Ken said:
They are used anywhere we need a small capacitance (obviously) even at
low frequencies.
Two that come to mind at relatively low frequencies are in the
compensation loop of a switchmode power supply where the dominant pole
is set by (primarily) the output of a current source, which means it
has a very high output resistance - the supply is switching at about
300kHz and the compensation cap is 33pF. (That's not the only
compensation component there).
Another is the loading caps on crystals (as part of a crystal
oscillator) as low as 1MHz (perhaps lower) - typical values for this
are in the range of 10pF to 33pF.
There are a lot of reasons for using small caps - as always, it's
application dependent.
Cheers
PeteS