Why 33pf capacitor is used in oscillator circuit of microcontroller?

Answer 1

Capacitors can be used for many purposes - for debounce of inputs, power isolation, curcuit loading. Some microcontrollers have D/A converters, oscillators, or transmitters built in and require external capacitors to operate these at specific frequencies.

Answer 2

Because that's what Intel specs as required in order to ensure proper operation.