It is easy to criticize the Rabbinical sources for inconsistency, as I did many times, but I would truly like to find a plausible and reasonable explanation for a certain trend in the Talmud based on my current learning.
Masechet Succah is dedicated to the Mitzvah of sitting in Succah (and 4 Minim). One would expect the subject to be covered from all possible angles. However, a very important Braytah and a whole discussion around it is brought in Masechet Erchin (3b) instead:
.הכל חייבין בסוכה: כהנים לוים וישראלים
A Baraita teaches: Everyone is obligated in the mitzva of Succah, including priests, Levites, and Israelites.
This passage discusses the obligation of Cohanim and Leviim in sitting in Succah (there's no such discussion in Masechet Succah). I understand completely, that the Baraitah is a part of a series of statements starting with "Everybody/Everything..." but that alone seems to be a bad reason to group Halachic topics if one wants to understand the Halachah as opposed to merely memorizing it. (This method of arranging discussion By-The-Way is worth a question on its own).
The Beraytah was clearly known to the editors of the Talmud, and nevertheless, they decided to put it in Erchin and not Succah (there's also a viable option of copying the passage several times).
Please help me to find a reasonable explanation of how the placement of this Beraytah in Erchin instead of Succah helps to understand the topic of sitting in Succah?
Do not comment "that's the way they did it back then" as we hold the Talmud eternal.
Do not comment "there are tens of other examples" as it does not help in the understanding of the phenomenon
Do not comment "the purpose of learning Torah is to merely engage in reading/murmuring Talmudic passages", as this question exclusively addresses the goal of understanding the Halachah
Do not comment "it proves one should be proficient in the whole Sha"S" as it is just a huge drawback caused by a decision to scatter the subjects over the Talmud