Why we use candles:
Any tools used in spell work such as candles are simply focus aids.
There are various reasons we use candles, oils and herbs in our spell castings.
Certain colors help us associate with certain goals, so we use candles of a particular color to help focus on the goal. in mind (Such as green for growth for one example.)
The candle flame represents the earth element of fire. We can draw on this elemental energy in spell work.
Some simply like to focus on the candle flame as a way to meditate and prepare to cast a spell, to clear the mind and focus on the task at hand.
Why we use herbs.
Herbs is made from the earth and helps to male any spell casting even more powerful, by using them with the candles
Why we use Oils:
Oils just helps to add to the herbs makes the spell stick to the goal in mind.