In this sugya we learn about the obligation of Birkas Hamazon. We discuss the various sources that this chiyuv is derived from. We also discuss whether the chiyuv is dependant on whether one has eaten, or whether one must feel "satiated".
This distinction has several Halachic ramifications - for example, a Bar Mitzvah boy who eats a meal right before he turns Bar Mitzvah, but still feels full when he becomes a Gadol. This also ties in to the Manna, which was eaten for sustainment but may not have provided the feeling of satiety. What if one ate and then vomited - is he obligated to to say Birkas Hamazon? Also discussed in this sugya, is the special Bracha on smelling , as there is the pleasure from smelling but no satiety - which may be why there is no Bracha Acharona on smell.
One of these sources is found in Parshas Beshalach, when Moshe Rabeinu instituted this practice to thank Hashem for the Manna as mentioned in the Gemara Brachos 48b.