Question:
The MB seems to be saying that there is no need “whatsoever” to be stringent and eat meat and fish (impromtu) in the sukkah.
How does that fit with the words of the Shulchan Aruch (ibid.) that it is praiseworthy to refrain even from drinking water outside of the Sukkah?
Answer:
What the Mishna Berura is referring to is the basic halacha, to that he says that there is no halachic need to be stringent on this. What the Shulchan Aruch is saying is that if someone will be machmir more than the halacha, and not eat or drink anything outside of the succah that it is praiseworthy. It is not clear however why it is praiseworthy. See Pischei Teshuva D:H Umi Sheyachmir.
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