Purposes: Chakra alignment, kundalini healing, hara (Heart Chakra) connection. This Symbol is used during the final stage of the Reiki Attunement process intending to ground and seal the newly awakened Reiki energies. From the heavens to the Earth, the lightning bolt Symbol is drawn.
Raku (rah koo) is another added Symbol that is often used to end Attunements (and sometimes to begin them) I have seen Buddhist and Shinto priests use this Symbol as a gesture to end Rituals so believe it was probably adapted from these for use with Reiki. Most often Raku is only used in Attunements. To open and close and ground the aura there is no need for additional Attunement to use these Symbols anyone who is attuned to Reiki Master can draw on these functions if they wish to do so. Raku is an addition to traditional Reiki I have not found it to be of any real value myself but your experience might differ.
Raku is used in Attunements, not in individual healings. It helps lift negative energy while taking the initiate into awareness. Raku activates the Hara Line, bringing Reiki energy in through the chi channels, and last settling in the Hara. It is used at the end of Attunements in both Usui and Karuna Reiki Attunements. Last, it is used to separate the Master from the initiate after the Attunement is complete.