Looking through my blog stats, I noticed that someone had searched on this and found a link here. So, I thought I’d give a little more answer to the question.
1) Symbolic usefulness:
In a number of traditions (including the one I work in), it’s used in part to represent the elements of air, and more specifically the elements of air and fire during the blessing of the circle. (Water and earth are represented with saltwater.)
Incense is a lovely way to mix these: it’s also something that can be carried around the circle and shared with everyone there.
2) Scent is an extremely powerful trigger.
Some scents bring us back to our childhood. Others remind us of a specific person, or setting. Scent seems to get far into our hind brain, in ways that we’re still learning about scientifically.
Some groups use the same scent for every ritual – simply smelling it helps trigger everyone into ritual mindset. Other groups use appropriate scents for a ritual (so Beltane might involve flowers, and a meditation ritual might include sandalwood.)
The shiny new group is looking at the consistent scent approach, but we’re still figuring out what that might be.
3) There’s something very alive about it.
Watching the tail of smoke rise and shift is – like the candle flame – a moving, changing thing on the altar, transforming as we do things. Nifty.
One of the reasons I like incense is that it has all three of these things: some of the other options (a feather, or a oil burner) don’t do all of these the same way.