Some incense require charcoal to burn. In particular the older style of incenses which have no additives to keep it burning. So you might say the more natural the incense the more likely you will need charcoal to burn it. Self lighting or ez light is best as it requires contact with a flame to easily ignite.
Resin incense requires charcoal to burn as it has no other medium within it to allow it to burn. Putting a flame to resins is just a waste of really nice incense, as it will simply burn up and not melt and vaporize releasing it's incredible essence.
Charcoal for incense is a very clean charcoal, it does contain Salt Peter for the quick starting type. Once the charcoal disk is ignited it will be ready for use on a bout 3-5 minuets. Bear in mind there are many companies that make this type of charcoal an not all are created alike. Some are better than others in that the charcoal will last much longer than others.
To use charcoal a heat resistant burner or container must be used the ideal container if ceramic must be kiln fired to with stand the heat and not crack. A crystal ashtray WILL Explode from the heat. It is best to place an inch or two of sand in the bottom of a bowl or burner to keep the bottom of the container from getting too hot to handle and to protect your furniture. Please be responsible and never leave an ignited charcoal unattended.
After the charcoal turns gray, feel free to apply resins, herbal incense , or what have you to the charcoal. You can always add more but what ever you put on is committed to burning. If you try to pick it up you'll get burned as the resin liquefies and will become like hot liquid varnish to your skin. So enjoy the fragrance and the properties that what you are burning has and relax.
Charcoal makes burning herbal incenses easier in that you eliminate the need to fan it to keep it going. Smudge such as cedar and sage burn so much better using easy light charcoals. By easy light we mean charcoal that self light when a flame is touched to the charcoal. Incense charcoal just makes burning easier and gives you more time to focus on ceremony rather than keeping things smoldering!