In Bamidbar chapter 7, around verse 19, Rashi suddenly turns uncharacteristically "drushy". He starts bringing all kinds of gematrias about the korbanot that the nesiim bring. But the gematrias don't seem to have anything to do with the topic at hand in the Torah. Rashi normally doesn't depart much from the level of p'shat, so it is surprising to see him start making drashot--especially when they do not seem to be related at all. Why did Rashi bring these seemingly unrelated gematrias?
Rashi is looking to explain why the continued repetition despite the lack of variation each time together with the idea that a single tribe gave the idea. He explains this by saying each tribe had a different intention, but that each individual tribal intention is related to the general intention established by the tribe that gave the idea.
The Lubavitcher Rebbe addresses this question in Likutei Sichos 8, page 41 and further. It is a somewhat long explanation, and I realize that many readers of this site are not familiar with Yiddish, so I'll try to summarize to the degree that I have time. This is translated into Hebrew for those interested, although it is not available online.
First he brings the answer of Maharal that Rashi is looking to explain his previous statement, that that Bnei Yissachar gave the idea of these specific Korbanos - in other words there was something special about this specific selection of Korbanos, so he explains what they are - what was so special about this advice.
The Rebbe has difficulty with this answer because it doesn't explain why Rashi prefers the drashos of Rabbi Moshe Hadarshan instead of the Medrash - especially the Medrash which gives one explanation for the intention of all of the tribes, which also has the advantage of being closer to Peshat than what Rashi brings.
He then brings the Reim (רא"ם) who argues and says that the reason is that Rashi is looking to explain why these specific sacrifices were chosen. Besides the failure of this reason to answer why Rabbi Moshe Hadarshan's explanation is chosen, it further raises the problem of why Rashi would say it on the second set of Korbanos, and not the first.
He also asks a general question on both explanations that in Peshat, you understand that there are deeper levels, you just don't need to explain them. If so, why does this particular choice of Korbanos need a specific detail drush to explain them, whereas other things are just left as is to be explained at deeper levels not brought by Rashi.
Therefore he explains that Rashi is looking to explain the following:
The student asks himself a question: why is it that the Torah repeats the Korbanos 12 times in a row without change? And he answers to himself that each tribe had different intentions so it can't be said that they brought the same thing. However, that raises some problems:
This is answered by the specific Drashos of Rabbi Moshe HaDarshan which show how one general intention can itself be divided into different specific intentions. Rashi is explaining the general intention. The Rebbe goes at length through each one comparing it to the alternatives in the Medrash and explaining how each choice best represents that idea.
In Likkuet Sichos 18 page 83 the Rebbe revisits this question and further explains how all of these Drashos are interrelated.