Why do you put quartz in watches?

Quartz usually gets used in two possible ways in watches.

The most common way quartz gets used in watches is in the form of a quartz crystal oscillator. When a crystal of quartz is properly cut and mounted, it can be made to distort in an electric field by applying a voltage to an electrode near or on the crystal. When the field is removed, the quartz will generate an electric field as it returns to its previous shape, and this can generate a voltage. The crystal can thus be made to oscillate (vibrate) with a consistent characteristic frequency. Quartz has the further advantage that its elastic constants and its size change in such a way that the frequency dependence on temperature can be very low. In this way, the crystal can produce a dependable signal of known fixed frequency and thus can be used to measure time. Analog watches take this signal and use it to regulate the speed of movement of the hands of the watch. Digital watches can use the signal more directly to regulate when the display changes.

Another more obscure application is the use of quartz crystal instead of glass to cover the watch face. The crystalline structure of the quartz makes the appearance a little nicer than when glass or plastic is used - and the quartz is a bit harder and more durable. It also is a bit more pressure resistant so it will occasionally be used this way in dive watches. Very few watches actually do this of course.