One cleaves to Him, may He be blessed, through the letters of Torah [study] and prayer, by cleaving one's thoughts and inner [being] to the inner spirituality within the letters. [This is the hidden meaning] in the secret, "May He kiss me from the kisses of His mouth," (Shir HaShirim 1:2) a cleaving of spirit to spirit. And this is as I heard (from my teacher [Achiyah the Shilonite]) [regarding the verse (Tehillim 68:14)] "If they will dwell between the lips[....]"
~ Keser Shem Tov, sec. 44a
The letters are vessels for the Divine Light. It is explained in Sha'ar HaYichud that the 22 letters of the Alef-Beis correspond to the 22 different effluxes of life-force through which the universe was and is created. However, their light is hidden, since one is not instantly overwhelmed upon perceiving the G‑dly energy within them. To reveal that light, one must bind one's thought to the letters, rising from level to level, world to world, and height upon height until the Highest of heights.
The letters of Torah and prayer, though certainly holy and revelatory, also conceal an infinite depth of Light hidden behind veils without number. But if so, what of the simple Jews who don't know of these mystical intentions? How can they cleave to the Eternal King?
When embracing a king, it doesn't matter whether he is robed in many garments or few. Even if he is covered by many garments, by embracing the garments, one is also embracing the king who is wearing them (Tanya, end of ch. 4). By cleaving to the letters of Torah and prayer, one cleaves to Hashem Himself.