One possible answer is due to the lights of the menorah (25:37). Was there a need to have lights in the mishkan? One might answer that it is obvious, people need light to see. However, from Shemot 27:21 and Vayikra 24:3 it seems that the lamps were lit only at night since apparently during the daytime, some sunlight was able to provide light inside the mishkan through the one side where there was only one veil, 26:36,37, as most likely, the veil was not that thick to stop light from passing through. (Note, the Talmud, Shabbat 22b, quotes Rav that the western light on the menorah was used to light the other lights because this light burnt all day, and apparently this was the practice in the Bet ha-Mikdash, see Ramban on 27:20.)
Was anybody in the mishkan at night? The Torah never records anybody entering at night, but maybe when the menorah was lit at twilight (30:8) there was a need for light just then. Another possibility is that the Abravanel (on 27:20,21, 1997, p. 462, also see N. Leibowitz, 1976a, p. 512) writes that Moshe was able to enter the mishkan at all times, and Moshe would have needed the light if he came into the mishkan at night.
Rashbam (on 25:31, quoted also by Bekhor Shor and Hizkuni) writes that the lights from the menorah were to illuminate the table. Yet, the Rashbam does not explain why there was a need to illuminate the table.
Rambam (Moreh 3:45, see also Amos Chacham, 1991, vol. 2, p. 280) writes that the lights were in order to glorify and honor the Bet ha-Mikdash. Apparently he believes that the light would enable people to see the rooms of the Bet ha-Mikdash and this vision would increase their awe of the Bet ha-Mikdash. (I believe though I am not positive that the Rambam was also referring to the mishkan.)
Another possibility is that the lights on the menorah symbolized the revelation of G-d, see Zechariah 4:6 and statement by Rav Sheshet in Shabbat 22b. This symbolism could be the reference to the statement in 27:21 that the lighting of the menorah was before G-d, see Amos Chacham (1991, on 27:21). This idea explains why there was a need for light, but the Torah records that the menorah had an intricate design. Do the details relate to the lighting? Were the details just for decoration?
N. Leibowitz (1976a, p. 500) notes that while the Rambam was unconcerned with the details of the menorah, many commentators believed that there had to be some lesson to be learned from these details. She quotes Abravanel who suggested that all the aspects of menorah relate to knowledge, and the Alshikh who relates them to moral qualities. However, she notes that allegoric explanations, such as these, are subjective. Yet, there is one approach that has been developed at least since the 19th century that accords with the simple reading of the text.
In a lengthy excursus on the menorah, S.R. Hirsch writes (1989, pp. 459,460), "If we picture its whole appearance, with its flowery base, its stem and arms with their almond shaped cups, knobs and flowers, the whole appearance strikes one as a tree,… blossoming upwards from its root,… a golden tree." Similarly, Benno Jacob (1992, p. 906) writes of the menorah that "its description… leaves no doubt that we are dealing with a unified organism which was to possess the form of a stylized tree." Likewise, Sarna (1991, p. 165) writes, "the shape of the lampstand - the trunk with its branches extending on either side - unmistakably evokes the image of a tree."
The idea explains why all the pieces of the menorah were to be made together from one block of gold (25:36), and not joined together after they were fashioned since the menorah was one unit like a tree. The base of the menorah is a yerech (25:31), which would be like the roots of the tree. The word the Torah uses for the main shaft and the sides going up is a kaneh, which is a reed. The seven kanim were decorated with gaviaim, kaftorim and perahim, all of whom relate to flora. The gavia is referred to as being almonds (25:33), the kaftor were apple shaped according to Rashi (on 25:31), and perahim are flowers. They would be like fruit on the branches of a tree.
Why was there an image of a tree in the mishkan? Various suggestions have been offered. Hirsch refers to the tree of knowledge, but I did not understand the connection. Benno Jacob suggests that the menorah parallels Aharon since he was the first in the family tree of priests. Sarna writes that possibly the imagery is to represent the tree of life, but he does not explain this relationship. Everett Fox (1995, p. 402) writes that trees "connote permanence, growth, and majesty- in other words a reflection of the divine."
Maybe trees symbolize a connection between man and G-d. A tree begins in the ground and grows upward towards the heavens. In the Garden of Eden, it was the trees and their fruit which were the bridge between G-d and mankind. When Adam and Chava ate from the fruit of the tree of knowledge, they became G-d like, Bereshit 3:22. If they had eaten from the tree of life, then they would have had eternal life, and again been similar to G-d, see our discussion on Bereshit 2:9,17 "Two trees."
The two ideas of light and the image of a tree can be combined, as together they give the image of a fire on a tree. This might be the imagery by the burning bush, Shemot 3:2. In addition, and this could be the reason for the burning bush, when Moshe went up on Mount Sinai to get the luchot, the people saw the glory of G-d as a burning fire, 24:32. This burning image would have been seen by the person who lit the menorah, and maybe by the people if they were able to see the menorah lit up from outside the mishkan.
In our discussion on chapters 26,27, "The three zones of the mishkan and the covenantal process," we suggested that the mishkan represents different aspects of the covenant. Every covenant has some type of revelation. By the covenant at Mount Sinai, the revelation was the people's vision of the glory of G-d (also 24:10,11). Accordingly, if the menorah was able to evoke that vision, then the menorah was also a symbol of the covenant.
Bibliography:
Abravanel (1437-1508), 1997, Commentary on Shemot, Jerusalem: Horev
Chacham, Amos, 1991, Da'at Mikra: Commentary on Shemot, Jerusalem: Mossad Harav Kook.
Fox, Everett, 1995, The Five Books of Moses: A new translation, New York: Schocken Books.
Hirsch, S. R. (1808-1888), 1989, The Pentateuch, rendered into English by Isaac Levy, second edition, Gateshead: Judaica Press.
Jacob, Benno (1869-1945), 1992, The second book of the Bible: Exodus, translated with an introduction by Walter Jacob, Hoboken: Ktav Publishing House.
Leibowitz, Nehama (1905-1997), 1976a, Studies in Shemot, translated by Aryeh Newman, Jerusalem: The World Zionist Organization.
Sarna, Nahum (1923-2005), 1991, The JPS Torah Commentary: Exodus, Philadelphia: The Jewish Publication Society.