Why do Gurbani

In a simple sense, it is to train the mind. The scriptures are written in a such a way it is always being read to the mind.

Below is an example how the words from the 9th Guru is directed to the mind.

The words "The ik-oNkaar satgur parsaad", it to tell your mind about the One and Only True God.

The idea is to get the mind to realise about the truth of God. A big percentage of this is about getting in touch with the subconscious mind.