Reasons why we use joss stick other than polluting the air

1. As an offering to God.
2. As a medium to send a message. Something like a letter but for God.
3. Gathering the spirits of the deceased. Probably when the gate opens during ghost month.
4. Calming our minds and body.
5. Getting rid of bad spirits and bad luck.
6. Praying for good health and prosperity.

Translated from http://pankuoooo.pixnet.net/blog/post/165212832-%E7%87%92%E9%A6%99%E7%9A%84%E6%84%8F%E7%BE%A9