The Upanishads, whose main concern is with transcendent Reality, have perforce to employ symbolic language. Brahman stands for a vastness and brightness free of form, feature or colour. The Reality can only be known through direct perception, through self-identity. Hence the abundant use of pronouns for understanding God in Hinduism.
The Upanishad says, "That which this Purusha is, That I am." In this sentence of eight words, five are pronouns. How else can the identity of the self and the Self be conveyed?
The Kena Upanishad starts with the pronoun Kena (By whom) and repeats again and again the formula: "That is Brahman by which the voice speaks, the mind thinks, the eye sees, the ear hears, That, and not what is worshipped." In other words, Brahman is the awareness behind our mind and senses and hence unknowable to our mind and senses.
Eleven times the Katha Upanishad repeats: Etad vai Tat" (This indeed is That), the ultimate is the intimate, the universal is the unique and vice versa. Likewise, the Chandogya repeats eight times the famous utterance: "Tat tvam asi" (That thou art).
These pronouns serve to bring the mystical into our ordinary experience and our common language.
Source – An excerpt from the article titled Vedic Symbol And Language by Dr. R. D. Nirakari in the page 6 of January 1975 issue of The Mountain Path.