Diksha is the giving of the mantra by the guru. Initiation bestows spiritual knowledge and destroys ignorance. As one lamp is lit by the flame of another, so the divine shakti consisting of mantra is communicated from guru to disciple. Just as electricity flows from a positive charged battery towards the negative pole, even so the power, energy, love, wisdom and spiritual current flow from the master-mind of a yogi towards the lesser mind of the disciple.
Guru is present in the mantra which he enlivens and communicates. The mantra is the body of the devata. The guru is the embodiment of the deity that is invoked. The purity of the mantra arises by properly knowing it from the preceptor. A certain spiritual vibration of the satguru is actually transferred to the mind of the disciple with initiation. Sri Ramakrishna Paramahamsa actually transferred his spiritual power to Swami Vivekananda. Lord Jesus did the same to his disciples. This is the masters touch.
Initiation is necessary to go along the spiritual path. Guru shows you the path. When you are initiated, your body and mind become purified. Initiation tears the veil of mystery and enables the disciple to grasp the hidden truth behind scriptural statements which are generally veiled in mystic language and cannot be understood by self-study. The guru will give you, by diksha, the right perspective in which to study the scriptures. He will flash his torch of self-realization on the truth which you will grasp immediately.
Yoga should be learnt from a guru, only then you will be able to understand its subtle points. Get practical lessons from a guru. Then alone you will grow rapidly. The guru will remove your doubts and show you the right path, because he has already trodden the path himself. It is the guru who will recognize the class to which the aspirant belongs and prescribe the suitable sadhana. Nowadays aspirants have the dangerous and wrong notion of imagining that they are highly qualified to adopt the highest form of yoga in the very beginning of their sadhana. This is the reason for the early downfall of the majority of aspirants. Such an attitude itself shows that the person is not yet ready to take to yoga. The real, qualified aspirant will be humble enough to approach a guru, surrender himself to the guru, serve the guru and learn yoga from the guru. For that which quickens self-realization and bestows awareness is initiation from guru.