The idea of the witch bottle is that it is a non living representation of the witch. The purpose is to deflect magical harm
from the actual witch and draw negative magics toward the bottle. Think of it as trying to confuse the magic as to the target of a working directed at the witch.
A bottle will contain elements that personalise it as the witch. Common inclusions are hair, toenails, urine and menstrual blood. With this in mind you can perhaps understand why it is common practise to bury the bottle. No witch would want their personal taglocks falling into the hands of strangers. The burying (hiding) also helps to confuse the location of the target as the perpetrator of the working doesn't know where the bottle is located.
As well as the personal taglocks of the witch, other items stored in the bottle often include things with properties of protection or magical deflection (agrimony being the herb that springs to mind). For those following a more hoodoo oriented path brick dust and graveyard dirt are commonly used.