The clear, healing tone of a bell can have a few different effects. The bell's vibrations can banish unwanted spirits and/or negative energies, call in positive, desired energetic influences, and/or simply clear stagnant energy in any space throughout ones home. Sound affects energy on a physical level - because sound is energy - and so the bell is both a physical and symbolic tool of the witch.
Bells are also used as part of many traditional witchcraft practices, you may choose to keep a bell on your altar and use it in your formal rituals. The bell is usually on the left side of the altar, where tools representing the goddess are found. In most traditions, it is associated with the element of air, though some attribute it to water because of the way sound waves ripple outward when the bell is rung.
In ritual, the bell may be used to invoke the goddess, and/or the elements. Some will ring it after casting the circle to seal the energy within, while others will ring it after releasing the circle to disperse any remaining energy. It can also be used to mark different sections of a longer ritual, such as the end of the invocations and the beginning of the main body of the ritual. Bells also make a lovely way to seal different kinds of spellwork. Outside of ritual, many witches like to hang one on the front door to guard their home.