A frequency comb is nothing else but an oscillator that is not producing a continuous sinusoidal wave, but it is producing periodic pulses of sinusoidal waves, and which also satisfies the additional requirement that there must be a precise ratio between the frequency of the sinusoidal signal and the repetition frequency of the pulses.

It is simple to make a pulsed oscillator, but making one where the pulse frequency and the sinusoidal frequency maintain a fixed ratio is not at all simple, especially when the frequency ratio is very large, like what is needed when the pulse frequency must be low enough to drive a digital counter and the sinusoidal frequency must be in the optical range, up to ultraviolet light.

If you have such an oscillator, by tuning the low frequency you can obtain light with an accurately known frequency. Alternatively, by tuning the high frequency to match light with a known frequency, e.g. produced by an optical atomic clock, you can obtain a pulse train with a known frequency, which may be used as a reference frequency, e.g. for a digital clock.