Why do we use VCO and not fixed frequency oscillator?

Excuse me, the oscillators with fixed oscillation frequency usually have relatively higher Q factor, so why do we use VCO?

The answer is in your question.

One is controlled and one is fixed.

VCO used as a part of many circuits, for example PLL

I think my question is not clear enough.
I guess it is because tunable filter is not easily realized, we use VCO to get a fixed output frequency although the input signal is of different frequencies.

I don't understand you, know.

But i think Old Nick's answer is Ok.

totally application specific

Well I can give one example for you ... you have to understand mixing and intermediate frequencies to get it:

I've just worked on a project where we used a magnetron as a source. The frequency drifts with temperature changes. The source is mixed a few times and filtered (A SAW filter, very high Q). If the source drifts too much then when we mix, the resulting signal does not make it through the passband. The only thing we could do was design a PLL with a VCO so we could change intermediate frequencies, which would bring the resultant back into the passband.

Hope this isn't too complicated.