In the Mishnah reclining actually plays a halachic role in any meal eaten on any day of the year with respect to the requirement to recite bircas hamazon with a zimun a quorum of three (or more) adult males. The Mishnah (Berachos 6:6) rules “If they sat to eat each one blesses for himself; if they reclined one person blesses for all of them.” The halachah is that only a meal with kevius in which each of the participants has a set place at the meal may be concluded with a zimun — and that this kevius is established by haseibah (Rashi Berachos 42a; Rambam Peirush HaMishnayos ad loc.; Rav Ovadiah Bartenura ad loc.). As the Gemara (Berachos ibid.) infers if the three participants do not recline while eating no zimun may be recited.

With respect to this requirement to recline however the Gemara itself (ibid.) allows for a more flexible definition of the required kevius; other conditions such as a prior arrangement to sit together can serve in its place. On that basis many Rishonim rule that in the post-Talmudic era with its altered norms of eating the convention of sitting at one table can create a kevius paralleling that which would be created by haseibah. While the Shulchan Aruch (Orach Chayim 167) does cite the requirement to recline in order to recite a zimun it also notes that the requirement no longer exists because the standard manner of eating has changed.  The requirement to recline at the Pesach Seder however appears in Shulchan Aruch (Orach Chayim 472) with no qualification or distinction.  Apparently the shift in common dining practice does not justify eliminating the requirement of haseibah on Pesach night. But why should this be so?