Mere theoretical knowledge does not make one a Guru. A Guru possesses tremendous divine powers. A Guru is the human ideal of perfection. To light a candle, you need a burning candle, likewise, an illuminated soul can enlighten another soul. Not only to learn meditation, not only to learn mantras, not only to learn one's religion, but to be able to discriminate between right and wrong, to love each and every living being, and to take life easy as it comes to us, a Guru is absolutely necessary.
Guru can cause self-illumination.
Guru's presence can be inspiring. Guru can give spiritual education. Guru can mold your mind to control your senses. Guru can remove the pitfalls and obstacles encountered while doing meditation. Guru can help eradicate egoism, hatred, prejudices, selfishness, and other negative qualities.