Why is a guide necessary? Because you can easily be lead astray. In prayer, you may experience visions or lights which are demonic and lead you to pride. A guide can help you deal with such such experiences. He can help you avoid being trapped by your pride and self will. He can guide you in a progressive way, growing step by step to the stage where you will see the true light.
Saint Porphyrios speaks of the danger of delusion,
And if in this spiritual dimension desire is enkindled, not by your good self, but by the other self, the egotistical self, then undoubtably you will begin to experience a pseudo-joy. But in your outward life you will be ever more aggressive and irascible, more quick-tempered and fretful. These are the signs of a person who is deluded.
A guide must be a person who is experienced in the prayer of the heart. It cannot be someone who prays mechanically and has not experienced prayer with the grace of God. Such a person will only be able to tell you what he has read in books.
A spiritual guide will keep you out of danger of delusion.
The Saint advises,
This is the teaching. We say that prayer cannot be taught, but in point of fact it can be taught when you live with someone who truly prays. When you take a book about prayer and read it, it may be that you don't understand anything. But when you have an elder next to you who prays, whatever he tells you about prayer you understand and take to heart.
Reference: Wounded By Love, pp 124 - 126