RASHI RULES
1. Rashi is "Jewish Jeopardy" -- he is answering a question (which he does not usually state).
2. Rashi always brings a pshat (contextual) answer, unless forced to bring a Midrash or other non-literal interpretation.
3. Rashi only quotes the name of the author of a statement if it helps us understand the meaning of the text.
וארגמן v'argaman — wool coloured with a kind of dye the name of which is ארגמן
tanned ram skins, tachash skins, and acacia wood;
ועצי שטים AND ACACIA WOOD — And from where did they get this in the wilderness? Rabbi Tanchuma explained it thus: Our father Jacob foresaw by the gift of the Holy Spirit that Israel would build a Tabernacle in the wilderness: he therefore brought cedars (acacia) to Egypt and planted them there, and instructed his children to take these with them when they would leave Egypt (Midrash Tanchuma, Terumah 9; cf. Bereishit Rabbah 94 and Rashi on Exodus 26:15).
ועצי שטים. ...כי היה סמוך אל הר סיני יער עצי שטים...
...there was a forest of Acacia trees near Mt. Sinai, [and they chopped them down for the Mishkan].
ועצי שטים. ...ובמדבר היו שם יערים גדולים שנטלו משם אותן עצים הקרוים שטים דכתיב וישב ישראל בשטים.
ועצי שטים, “and acacia wood.” ...There were some forests in the desert from which the Israelites were able to cut boards which they called shittim. This is also why we read in Joshua 2,1 that “Joshua sent out spies from (the forest around Shittim)."
Why didn't Rashi (source 8) give a simpler answer, like Ibn Ezra (source 9) and Daas Zekeinim (source 10)?
(א) פישון. הוּא נִילוּס נְהַר מִצְרַיִם.... דָּ"אַ פִּישׁוֹן, שֶׁהוּא מְגַדֵּל פִּשְׁתָּן, שֶׁנֶּאֱמַר עַל מִצְרַיִם (ישעיהו י"ט) וּבֹשׁוּ עֹבְדֵי פִשְׁתִּים:
(1) פישון PISHON — This is the Nile, the River of Egypt... Another interpretation of the name Pishon is: it is so called because it grows flax (פשתן) [i.e., in Egypt].
ועוד יכלול ספור המסעות תועלת אחרת כי ירמוז לעתיד, שהרי דברי הנביאים כולם מוכיחים שהגאולה האחרונה כדמיון הראשונה, וכשם שיצאו ישראל בגאולה ראשונה ממצרים אל המדבר כן בגאולה האחרונה עתידים שיצאו הרבה מישראל אל המדבר ויעברו במקומות האלה והקב"ה יכלכלם וינהלם שם כמו שעשה לישראל במדבר, והוא שאמר הנביא (יחזקאל כ׳:ל״ה) והוצאתים אל מדבר העמים, והכתוב הזה מדבר לעתיד בגאולה אחרונה.
Still another purpose of the Torah providing us with all these details about the Israelites’ journeying is that it contains allusions to the future. All the predictions of our prophets concerning the redemption of the future clearly indicate that this redemption will largely reflect earlier redemptions. The more we know about the redemption from Egypt, etc., the better we can picture how the redemption of the future will develop. Just as when the Israelites departed from Egypt they marched into the desert, so in the final redemption many Jews would also march into the desert on their way to their home...