There are two Mishnayot on today’s daf that are almost identical. In both of them, the scenario opens with three brothers, two of whom are married to sisters, while the third brother is married to an unrelated woman.
In the first Mishna, we learn that if one of the sisters becomes widowed and fulfills the mitzva of yibum with the brother to whom she is permitted (she cannot marry the brother who is married to her sister), should that brother now die with no children, neither she nor her tzara (fellow wife) will become yevamot to the surviving brother. She cannot because he is married to her sister; her fellow wife cannot because she is the tzara of a woman forbidden to the yavam.
In the second Mishna, it is the brother who is married to the unrelated woman who dies, and his wife fulfills the mitzva of yibum with one of the surviving brothers. Should that brother die, as well, neither she nor her tzara (fellow wife) will become yevamot to the surviving brother. She cannot because she is the tzara of a woman forbidden to the yavam, since he is married to her sister.
The Gemara asks the obvious question – why do we need both of these Mishnayot? Aren’t the cases so similar that there is no need to repeat the rule a second time?
The Gemara’s response is that we really do not need both, and that the rule of the second Mishna can be understood from the first. Nevertheless, once it was taught, Mishna lo zaza mimkoma – the Mishna is not moved from its place.
This idea is connected with a time when the Mishnayot were truly an oral tradition, and students learned them by heart in order. Leaving out or combining Mishnayot may have been very confusing to those students, so Rabbi Yehuda HaNasi (who edited the Mishna) chose to leave in Mishnayot even if they were not essential, since they were part of a long-studied tradition.