Smudging is an ancient way to purify, protect and lighten energies. It has been used by Shamans and Priests since fire was invented (Probably!).
When you use a smudge stick you are activating your will (fire) power to reach out to a higher level using a the smoke of a sacred herb (air).
These have risen in popularity over the years because they are so easy to use and they are also powerful and will accomplish your original intent: purification, protection and visions.
There is a huge variety and the more you work with Smudging sticks the more balanced you feel.
It really clears away the negative energies that can build up. It keeps you clean from those negative vibe and helps to keep your energies on a higher vibration rather than being dragged down by little nasties.