In the case of Reiki, the same tool set can be used for two different intentions: Healing or spiritual growth. Healing of different types comes with spiritual growth, although it is no longer the primary focus, but spiritual growth does not necessarily accompany healing. It is a matter of intention.
For the Tantric practitioner, Reiki study provides a starting place and a foundational tool for further advancement. You learn about the layers of body, emotions, mind and spirit. You learn to perceive energy, and increase your intuitive awareness of increasingly subtle layers of energy.
Reiki attunements boost the amount of life force energy available to you and working with Reiki on a regular basis further increases your capacity to hold and work with this energy. You learn about energy structures such as chakras and nadis. The more you work with the Reiki energy, the more your energetic body becomes cleansed and purified.
Each of these lessons and benefits fits directly into Tantric study and practice, where the practitioner is working to increase levels of energy in the body and mind and use this energy to purify all levels of energy structure and awareness. What Tantric practitioner would not want the boost of energy that Reiki provides, since that is part of the Tantric process anyway? What Tantric practitioner would not want a simple tool for balancing energy and increasing awareness?
Tantric study deepens your understanding of Reiki, as you are provided with additional tools for purification and deepened awareness. As you progress in Tantra, you work with layers even more subtle than just the body, mind and spirit as Reiki presents them, so Reiki presents a foundation for more advanced studies.
The deeper you go in Tantric study, the more you are able to transfer that new depth into the Reiki system as either a healing method or a spiritual advancement technique. You also are able to see the Tantric implications of the Reiki system more clearly. In fact, these two systems will likely merge into one thing.
Both Reiki and Tantra work together to deepen your awareness of energy and increase your capacity to hold energy while at the same time purifying your body and mind. Reiki also plays its part in helping to prepare the Tantric student for the more advanced practices such as Tantra Kundalini Yoga and Tantric Sacred Sexuality, where perception of subtle energy and energy structures is a must for accelerated progress.