The Gemara on our daf tells of the unusual case of Rabbi Hanina, who would take a bite out of his etrog and then use it to fulfill the mitzva. The explanation for his behavior is given by the Gemara as distinguishing between the first day of Sukkot, when there is a biblical obligation to take the daled minim (the four species) and other days of the holiday when the requirement is only of Rabbinic origin. When there is no biblical obligation, even an etrog that is haser – where a part is missing – is considered kosher.
This distinction helps us understand why Rabbi Hanina was able to fulfill his mitzva even though his etrog had a bite taken from it, but we still are at a loss to understand why Rabbi Hanina chose to do that – and from the Gemara it appears that he did this on a regular basis. This question is raised by the Meiri, who points out that it is odd that one of the Sages would choose to fulfill the mitzva this way on a regular basis, even if it was technically permitted to do so. He explains that Rabbi Hanina certainly said the blessing on a full etrog. Nevertheless, his tradition was to walk around holding a lulav and etrog in his hands throughout the day. In the course of the day he became hungry and took small bites from the etrog. Still, the Gemara feels that we can conclude from this story that such an etrog can be used for the mitzva after the first day.
Another issue raised by the commentators is that the daled minim are set aside for the mitzva, which makes it forbidden to use them for other purposes throughout the holiday. The Ritva (among others) suggests that Rabbi Hanina purchased a different etrog for each day of the holiday, so none of the etrogim were “set aside” for use on a particular day, thus once he fulfilled the mitzva for the day he was well within his rights to derive benefit from them.