Why does Rashi preface his comments by saying "The Medrash Explains This Well"?

Question

The first Rashi in Parshas Korach contains an introduction, where Rashi says that the Medrash explains this section well:

ויקח קרח. פָּרָשָׁה זוֹ יָפֶה נִדְרֶשֶׁת בְּמִדְרַשׁ רַבִּי תַנְחוּמָא:

ויקח קרח — This section is beautifully expounded in the Midrash of Rabbi Tanchuma.

Beginning with the second Rashi, Rashi proceeds to quote the Midrash and continues onwards with the explanation of the Parsha.

This seems to beg the question: why does Rashi have to preface his commentary by saying this is expounded well? It sounds almost like Rashi is saying "Hey, just giving you heads up, we have a good Pshat coming up". Do we really need that introduction? I would assume that if Rashi brought something as part of his commentary, then it would be a good commentary. Why do we need him to explicitly say this is a good commentary?

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Inspired by a (now deleted) comment on this question.

Answer 1

This is addressed by R. David Halevi Segal in his supercommentary on the spot:

לכאורה קשה שיש איסור בלשון זה שהרי ארז"ל שאסור לומר הלכה זו נאה הלכה זו אינה נאה דא”ל דדוקא תרווייהו יש איסור לומר ז”א דהא הלכה זו אינה נאה לחוד יש איסור אלא ע”כ דה”ק כשם שאסור לומר הלכה זו אינה נאה כך אסור לומר הלכה זו נאה דמתוך זה משמע שהלכות אחרות אינם נאים ח”ו וא”כ למה אמר רש”י פרשה זו כו’ וי”ל דגם כאן יש מיעוט דבשאר מקומות יש ב’ דרכים האחד לפי פשוטו והשני לפי המדרש אבל כאן אין כאן אלא דרך אחד דהמדרש הוא פשוטו שאין כאן פשוטו רק המדרש הוא יפה נדרש אפי’ לפי פשוטו דאין שום פירוש על ויקח מה לקח לפי פשוטו

It is seemingly difficult since there is a prohibition of this language, for the Sages said that it is forbidden to say “this law is nice [but] this [other] law is not nice”. We can’t suggest that it’s only both [parts of the statement] that is forbidden to say, because the statement “this law is not nice” is forbidden on its own. Rather it must mean that just as it is forbidden to say “this law is not nice” [on its own] it is forbidden to say “this law is nice” [on its own], since within this it implies that other laws are not nice, heaven forfend. And if so, why did Rashi say “this section [has been nicely expounded]?

We could say that here too there is a limitation [implied]. For in other places there are two approaches: one is according to the straightforward meaning, and the second is according to the Midrash. But here there is only one approach, for the Midrash is the straightforward meaning since there is no straightforward meaning. Rather, the Midrash is nicely expounded even according to the straightforward meaning, since there is no explanation for “And he took” that explains what he took according to the straightforward meaning.

In other words, since this verse says that Korach took but it doesn’t say what he took, the only way to make sense out of the verse is via the Midrash which becomes the straightforward meaning of the verse. That is what Rashi is alluding to by saying that specifically here the Midrash expounded nicely.