Orgyen Tobgyal Rinpoche explains the importance of the yidam:
When we consider all the great masters of the Indian and Tibetan traditions, we find that in every case, their accomplishment came about through their practice of a yidam. They chose a deity and guarded that practice like their very life force, and on the basis of that complete commitment to the path of deity yoga, they practised the stage of generation, the stage of completion, and integrated these arriving at their final realisation of complete accomplishment and enlightenment.
But nowadays there are people who say, “Oh, what a lot of bother! Deities and mantra, I hate all that. I’m just going to meditate.” And they sit there, and close their eyes, and that’s what they call practice. They say, “I just want to do effortless meditation.” But as Dilgo Khyentse Rinpoche said, “Although there are people like that, I’ve not seen them gaining any sign of attainment.”
In one of the texts of Ratna Lingpa’s cycle of Vajrakilaya practice, there is a passage which recounts how on one occasion Yeshe Tsogyal asked Guru Rinpoche about the nature of kyérim practice. “Do we really need a yidam deity?”, she questioned. Guru Rinpoche replied, “If there is no yidam, where is the source of siddhis? Without siddhi, how could there be enlightenment?”
In fact, if we look at the great masters of the Indian and Tibetan traditions of Vajrayana practice, we find there is no-one who did not meditate upon a yidam deity.
In the Nyingmapa school, all the great vidyadharas have had a main yidam deity which they practised. If their practice was of a peaceful deity, it was almost always Vajrasattva. If it was a wrathful deity, it was almost always Vajrakilaya. Guru Rinpoche and Vimalamitra themselves attested to the fact that Vajrakilaya was their yidam deity.
Read more at :http://www.rigpawiki.org/index.php?title=Yidam