Oscillator phase noise will show up as noise sidebands on the signals
you're measuring. They'll limit your ability to use your spectrum
analyzer to judge the noise that's really coming from the signals. As a
very good and professional engineer who spent a number of years
designing some of the best commercial spectrum analyzers made, Wes most
certainly paid a great deal of attention to phase noise and dynamic
range, two of the most important potentially limiting factors of
spectrum analyzer performance.

Roy Lewallen, W7EL

Hans Summers wrote:

. . .
I suppose oscillator phase noise is also important. . .