advantages of PLL over simple crystal oscillator

Freq synthesizer is not the same like PLL?
maybe u want to compare between PLL to crystal?

A PLL is part of the most common sort of frequency synthesizer. A frequency synthesizer is like an infinite range of crystal, each of which can be switched into circuit. They also offer superior frequency stability as they are locked to a broadcast reference, rubidium standard or a 10 MHz crystal (which is the best choice of frequency for a crystal stability). They do suffer from spurious outputs and phase noise though. These unwanted products are kept to low level so synthesizers are suitable for most common tasks of signal generation.