(a) The Gemara implies that this Korban is brought in order to be eaten
before the Korban Pesach, so that the Korban Pesach will be eaten "Al
ha'Sova," while one is satiated.
But what is the source for eating the Korban Pesach "Al ha'Sova?" RASHI says
that eating the Korban Pesach while one is satiated makes the act of eating
into one which is more significant and meaningful. Eating while satiated
enables a person to better experience and appreciate the taste, and thus it
is a Hidur Mitzvah to make the Korban Pesach taste better. (The MECHILTA,
Parshas Bo, ch. 6, learns from the verse "Al Matzos u'Merorim Yochluhu" that
"the Korban Pesach is eaten "Al ha'Sova" but not the Matzos and Maror." This
may be describing the Hidur Mitzvah that Rashi is referring to.)
The reason this applies only to the Korban Pesach and not to any other
Korbanos which one eats might be because it is the only Korban that is
brought solely for the purpose of being eaten. The other Korbanos, even
though they are eaten, are brought primarily for Kaparah.
(b) TOSFOS (DH Lav) cites the Yerushalmi (Pesachim 6:4) that seems to
conclude that the requirement to eat the Korban Pesach "Al ha'Sova" is only
d'Rabanan. The reason Tosfos gives is because of a Gezeirah to prevent
people from breaking the bones of the Korban Pesach (which is forbidden by
the Torah). If a person would eat the Korban Pesach right away, he might
break the bones of the Korban while eating it due to his haste. Therefore,
the Chagigah is brought so that a person will eat it first and be satiated
by the time he eats the Korban Pesach. (The Mechilta, then, may be only an
Asmachta.)
(c) However, another opinion mentioned in the Gemara, that of Ben Teima,
understands that the Chiyuv to bring the Chagigah of the Fourteenth is
mid'Oraisa. It is learned from a verse that compares the Korban Pesach to
the Chagigah. TOSFOS (DH Lav) adds that even the Rabanan who argue with Ben
Teima (who do not compare the Chagigah to the Korban Pesach, and maintain
that it is eaten for two days, unlike the Pesach), also derive from a verse
(71b) the Chiyuv to bring the Chagigah. If so, they both must hold that the
Chagigah is an obligation, and not just optional, and that it is brought
even on Shabbos, and even when the Tzibur Tamei.
The Halachah, however, follows the opinion of a third opinion -- the Tana of
our Mishnah, who says that the Chagigah is not brought on Shabbos. His
reasoning for bringing the Chagigah of the Fourteenth is apparently as
explained above in (a) and (b).
(d) The RAMBAM (Hilchos Korban Pesach 10:12), however, cites a verse for the
source of the Korban Chagigah of the Fourteenth, implying that it is
d'Oraisa. On the other hand, he says that it is an optional Korban, and it
is not brought on Shabbos! (TOSFOS 59b, DH b'Arba'ah, also concludes that
the Chagigah of the Fourteenth is not Docheh Shabbos even if it is
d'Oraisa.)
The Rambam seems to understand that even according to our Mishnah, the
Chagigah of the Fourteenth is d'Oraisa. Why, then, does the Gemara say that
the reason it is brought is in order to eat the Pesach "Al ha'Sova?"
According to the Rambam, perhaps the Gemara is suggesting why a person would
*want* to bring it if it is not required of him, and it answers that he
benefits from it by being able to eat the Korban Pesach "Al ha'Sova." But
even without that reason, the Torah gives the option to bring it -- perhaps
in order that the Mikdash should be filled with Korbanos on Erev Pesach.