Why do Sikh's believe their holy book is important?

Guru Granth Sahib Ji (the Sikhs holy book) contains the actual spoken Words by the Sikh Gurus and thus those Words themselves are now the Guru of Sikhs. Furthermore, Sikhs consider Guru Granth Sahib Ji as the Living Embodiment of Guru for the same reason. Sikhs don't treat Guru Granth Sahib Ji as Holy Book but as a Living Guru. Also, the 10th Guru, Guru Gobing Singh Ji, announced himself that out final guru would be the Guru Granth Sahib Ji.