What is the purpose of a squelch function?

The squelch control on a radio is designed to quiet the receiver when no signal is present. Without squelch, an FM receiver would constantly output loud, fatiguing white noise (static) when no one is transmitting. When an incoming signal is strong enough to 'break' or open the squelch, the audio is unmuted so you can hear the transmission. Squelch has nothing to do with transmitting, which rules out reducing CW key clicks or eliminating parasitic oscillations. It also does not filter out impulse noise; that is the job of a noise blanker. Its sole purpose is to keep the speaker quiet until an actual RF signal is received.