Quartz crystals are used in watches because they are very stable and have a very precise frequency. This means that they can be used to keep very accurate time. The quartz crystal is cut into a specific shape and then placed in an electronic circuit. When an electric current is applied to the crystal, it vibrates at a very precise frequency. This vibration is then used to keep time.
Quartz watches are much more accurate than mechanical watches. They are also more reliable and require less maintenance.