The purpose of the symbol Cho-Ku-Rei is to increase the flow of Reiki energy through the practitioner, and thus increase the healing. It is used in nearly every healing situation. It is important to keep this purpose of increasing power firmly in mind though.
While Essential and Traditional Reiki use the same symbol to represent Cho-Ku-Rei, the two schools of Reiki draw them in opposite directions. Essential Reiki draws the symbol clockwise while Traditional Reiki draws it counterclockwise. This makes the individual Reiki user's esoteric belief's around spirals critical in determining which version will be the more effective.
Many people, like the Wiccans, associate clockwise motion and spirals with an increase in power and counterclockwise with a decrease in power. Others, like many North American Indian tribes, associate sun wise (clockwise) with good (Medicine) and anti-sun wise (counterclockwise) with bad (Medicine). Yet others associate one direction with 'normal' and the other with 'contrary' with no good or bad orientation implied. Another group thinks the earth's hemispheres you are in important in determining the effect of spirals. That is, within the Northern hemisphere, clockwise is generally associated with an increase in power and, within the southern hemisphere, the reverse is true, with counterclockwise being the direction of increased power.
If you are uncertain of your own beliefs in this area or have a strong sense of adventure, the Reikifire Ministry recommends you experiment with both versions and see which boosts your energy more effectively. This will give you a clearer idea of your beliefs around spirals and their use and power -- and how you can use them most effectively.
There is another consideration. In some cases, increasing power may NOT be the shortest route to healing. It may be that a decrease in power would be more appropriate. Diane Stein, in her book Essential Reiki, refers to one healing, involving cancer, where her guides directed her to decrease energy (that is, use the counterclockwise Cho-Ku-Rei). If I understand correctly what she says, she is referring to decreasing the energy of the cancer rather than decreasing the energy of the healing. This is a good example of using Reiki in a tightly focused manner over multiple Reiki sessions.
It also points our the wisdom of taking nothing for granted and always asking for guidance in your healing work.
Cho-Ku-Rei is also used to increase or restore the life force and thus the nutritional value of food, to increase the potency of medication and/or reduce the side effects. Essentially in any situation where a strong increase or decrease in energy, it is appropriate to invoke the Cho-Ku-Rei
The healing focus of the Cho-Ku-Rei is the physical body.