Jars have many uses, some more magical than others. For kitchen witches, jars are often used to hold herbs and ingredients, for others it may hold moon water, black salt, or small crystals. They are used for practical purposes, but also in practice.
Spell jars, originally called witch bottles, were first seen in 17th century England. These are an example of many aspects of witchcraft being combined. These jars can be created for any intention you can think of. After cleansing the jar, adding different herbs, crystals, and other items will create the intention. Carrying the jar or placing it somewhere for its intention, such as putting a protection jar in the car, is how they are "used". Once the goal of that jar has been accomplished, Wiccans believe burying it to return it to the earth is one of the best ways to get rid of it.