The principal benefit from opening the third eye is to know thyself.
The third eye is a effortless space between the eye brows and slightly up and in. It's a cave. There rests the pituitary gland and your knowing - your intuition. When you know who you are, you are free. Without knowing who you are you limited. Knowing who you are, you are unlimited.
To open the door to the third eye, one must be invited. You cannot just walk in, uninvited. To be invited is Grace.
With countless sitting in stillness and silence, you will become empty. You only become empty when you become void of ignorance. Ignorance is attaching to the body and mind. Everything you know of is let go of. When everything is let go of, what is left? Emptiness.
The invitation is accepted, emptiness meets emptiness. There is no door in emptiness. It is just open.