A candle is a tool, and like any tool, it does things uniquely well that other tools don't do as well!
Candles have many excellent and unique qualities. They provide a single focal point (the flame). They dispose of themselves (so you don't have to dispose of the remains of a spell as you would if, for example, you used a crystal or a piece of writing). They continue to "work" without your attention. They are timed (a dinner taper lasts eight hours, a tea light lasts 45 minutes, a 7 day candle...well, you get the picture). They come in a variety of colors so that color symbolism can be used. They're inexpensive and readily available. Wax is soft, so candles can be engraved and objects can be embedded in them. They can be scented.
Each of the above is a reason that a candle is a uniquely useful tool.