The Zohar is written in such seemingly earthly language on purpose, unlike The Study of the Ten Sefirot, The Tree of Life, and “The Preface to the Science of Kabbalah” (Pticha), which are written in the language of Partzufim and Sefirot. For beginners, it is very challenging, but as soon as a person immerses himself in “work for the Creator,” meaning that he wishes to see all that The Zohar describes as transformations within the soul, he clearly sees that this language is helpful to him and obliges him to keep visualizing spiritual actions and attributes.
One can examine himself: “Am I present in spirituality or not? Do I see only spiritual properties rather than faces and events, or am I still “falling” into some corporeal movie which means that I am clearly not present in spirituality?”
If any words from our corporeal world evoke earthly images in us, it is a sign that we reside in the material desire. If they awaken spiritual properties and sensations, it means we are present in the spiritual one. Quite simply, each of us judges either by the degree of his corrupted nature or his corrected state.
Hence, it is a very useful analysis to verify where we are. Moreover, when you are in spirituality, you will have difficulty talking about corporeal notions since all of them derive from spirituality. You will struggle to understand what another person is talking about because you will be seeing only the spiritual picture.
There could be many arguments as to why Kabbalists chose the language of the branches to write The Book of Zohar, and all of them will be correct.
From the 2nd part of the Daily Kabbalah Lesson 1/3/2011, “Introduction of The Book of Zohar,” Article “Yitro (Jethro)”