In the spiritual world omens are everywhere and there are no such thing as coincidences. You can use signs around you to guide your life and your workings if you only know how to interpret them. 7 day candles have long been used by rootworkers to read a specific working and determine its effectiveness, or warn of any problems or solutions. Just like reading tea leaves, the burnt candles can provide valuable insight.
The soot on a candle is one of the most recognizable results of burning a candle. While in the mundane world one might say this was merely soot, in the spiritual we know there is a message to be interpreted in that soot.