1) To warm meridians and expel cold.
Abnormal flow of qi and blood in the body usually results from cold and heat. Cold causes obstructed flow or even stagnation of qi, and heat results in rapid flow of qi.
Normal heat activates blood circulation and cold impedes its smooth flow. Since stagnation of qi and blood is often relieved by warming up the qi, moxibustion is the right way to generate the smooth flow of qi with the help of the ignited moxa wool. “If stagnation of blood in the vessels cannot be treated by warming up with moxibustion, it cannot be treated by acupuncture; blood stagnation caused by cold, should be dispersed by moxibustion.”

2) To induce the smooth flow of qi and blood. Another function of moxibustion is to induce qi and blood to flow upward or downward.

3) To strengthen yang from collapse. Yang qi is the foundation of the human body. If it is in a sufficient condition, a man lives a long life; if it is lost, death occurs. Yang disorder is due to excess of yin, leading to cold, deficiency, and exhaustion of the primary qi, characterized by a fatal pulse. At this moment, moxibustion applied can reinforce yang qi and prevent collapse. In Chapter 73 of Miraculous Pivot it says, “Deficiency of both yin and yang should be treated by moxibustion.”

4) To prevent diseases and keep healthy.
It is often said:
“If one wants to be healthy, you should often have moxibustion over the point zusanli [ST-36].”
In Notes on Bian Que’s Moxibustion, it says, When a healthy man often has moxibustion to the points guangyuan [KI-4], qihai [KI-6], mingmen [GV-4], and zhongwan [KI-12], he would live a very long life, at least one hundred years.”
The relationship of moxibustion to the herbal material used for moxa wool depends on the technique.
For example, when moxa cones are burned on the end of acupuncture needles, the constituents of the moxa do not interact with the skin and the local effect is that of transferring heat to the acupoint.

When a moxa pole is used, there is some transfer of active constituents to the skin, but not much; the effect is still mostly heat, but to a broader area.
When the moxa cone is burned directly on the skin, active constituents are transferred to the skin.
This latter method was the most commonly used traditional approach, as best as one can tell from the old literature.

When moxa is burned on top of another herbal material, such as a slice of ginger or garlic, cake of aconite, or mound of salt or powder of herbs in the navel, the heat will drive some of the ingredients of the interposing herbal material into the skin, but little of the moxa wool ingredients will penetrate.

Fresh ginger, one of the commonly used interposing materials, contains compounds similar to borneol and camphor (small holes in the slice of ginger or slice of garlic permit a little of the moxa vapors to penetrate).
Of course, with all moxibustion techniques, the smoke and vapors from the moxa are ultimately inhaled.
The modern smokeless moxa poles are intended to eliminate this otherwise sure route to getting moxa ingredients internally, and might defeat part of the therapeutic action.
The Chinese technique of applying moxibustion for an extended period of time (up to 30 minutes for a treatment session), assures that the patient inhales a substantial amount of the vapors and smoke.