Not always, but it does reduce the intensity of unwanted low frequency vibrations passing from the floor, through the mic stand and into the microphone.
Typical vibration sources are foot tapping, kick drums on the same floor or passing traffic. Solid concrete floors tend to pose less of a problem than wooden floors, but as shock mounts are now relatively inexpensive, it makes sense to use one if you can.
If a shockmount is not an option these types of low frequency vibration can often be removed with either a mics built in high pass filter, or by manually inserting a 80Hz, 24dB/octave filter into the mic signal path.