If you think about it candles serve many purposes in the craft but aren't limited to..
Candles help the practitioner to focus and center, also the colors and meanings give the magick that extra ommph!
Most practitioners utilize candles in their craft. If you think about it in candle magick the candle is basically a battery for the spell and letting a candle burn all the way out even after you yourself have finished the spell and closed the circle and whatnot, will basically be the countinuing of the spell and when its burned out the candle has completed feeding into the magicks just cast meaning its done and sealed.
But there are many ways to use candles in the craft, it is prefered that one snuffs a candle or pinches it out so that if they have a repetative spell that needs the practitioner to continue thourghout the week, for the energy flow will remain as it did in the before rite. Also if you have old candles you want to utilize, just light it and blow it out symbolically stating that whatever mundane uses for that candle no longer remain so now its just a plain new old candle technically ready for use in whatever spells they be needed.
Like for me I have alot of candles in storage and if I don't have the new candle color for a certain spell then you can either go with white (for white subsitutes all other colors) or grab one of your old spell candles and light it and blow it out, so the influences of the older spell are released from the candle. Now you can utilize that candle for the new spell.
Or if you feel like your wasting a candle by letting it burn all the way down then you can cut it in half or stick a pin in it for where you would like the candle to end the spell( either way would be symbolically burning the candle all the way down).
But if your utilizing spells pre-made by others then their rules may apply so that one can tune into that spells pattern and make it successful.