Why do the use the items they do such as Arshina, sandalwood, milk, yogurt, Honey, bananas etc?
The greatest and the highest Abhisheka is to pour the waters of pure love on in one's heart. The external Abhisheka with various objects will help the growth of devotion and adoration for God and eventually lead to internal Abhisheka with pure abundant flow of love.
Swami Sivananda explains the significance of offering various items during the Abhishekam.
The Abhisheka is also accompanied by the chanting of the appropriate hymns for Lord Shiva, Lord Narayana as the case may be. Recitation of the hymns purifies the mind and fills it with Sattva.
The Abhisheka water or other articles used for Abhisheka are considered very sacred and bestow immense benefits on the devotees who take it as the Lord’s Prasad. It purifies the heart and destroys countless sins. It should be taken with intense Bhava and faith.
When you do Abhisheka with Bhava and devotion, your mind is concentrated. Your heart is filled with the image of the Lord and divine thoughts. You forget your body and its relation and surroundings. Egoism gradually vanishes. When there is forgetfulness, you begin to enjoy and taste the eternal bliss of the Lord.
By offering Panchamrita, honey, milk, etc., to the Lord, thoughts of your body diminish. Selfishness slowly vanishes. You derive immense joy. You begin to increase your offerings unto the Lord. Therefore, self-sacrifice and self-surrender come in. Naturally, there is an outpouring from your heart, “I am Thine, my Lord. All is Thine, my Lord”.
The offering of the articles along with the hymns symbolize offering oneself and the worldly things to the Lord.
Kannappa Nayanar, a great devotee of Lord Siva, a hunter by profession, did Abhisheka with the water in his mouth for the Linga at Kalahasti in South India and propitiated Lord Siva. It is the mental Bhava that counts and not the outward show. Lord Siva said to the temple priest: “This water from the mouth of Kannappa, my beloved devotee, is more pure than the water of the Ganga”.
The view that worship and Abhisheka have a more esoteric inner significance than the offering of the gross objects is also echoed by many saints. For instance, Thayumanavar, a well-known Tamil Saivite Saint, mentions in a verse : (Verse 151 of Hymn 43 Parapara Kanni English meaning below) :
My Heart is your Temple. The Sweet thought and reminiscences I have of you is the Fragrance. Love is the Water. Oh, the Universal One, I invite you to accept my Worship.