We learn in sidra Ekev of the importance of giving thanks to God after a meal – “V’achalta v’savata u’vayrachta et Adonai Elohecha al Ha’aretz ha’tovah asher natan lach. When you have eaten and are satisfied, bless the Eternal your God for the good land which God has given you.” (Deut 8:7-10)
The Birkat Hamazon has a number of blessings according to the Talmud (Berachot 48b)–
Birkat Hazan – to praise God who sustains the world;
Birkat Ha’aretz, to thank God for everything, focusing on the Land of Israel;
Boneh Yerushalayim – the petition to protect and rebuild Jerusalem,
And finally HaTov ve’haMeitiv, general praise and thanks for God.
These, and the zimun, the general invitation when three or more eat together, are taken from the verse like this:
When you have eaten your fill, you shall bless – Birkat haZimun;
The Eternal your God – Birkat Hazan;
For the…land – Birkat Ha’aretz;
…the good (land)… – Boneh Yerushalayim;
That God has given you – HaTov ve’haMeitiv.
While there is some dispute about the zimun and the final tov ve’haMeitiv, there is general agreement that to fulfil the obligation we must make the three blessings, of God, and of the land, and of Jerusalem, and that these are biblical commandments. (Jerusalem Talmud: Berachot)
Yet earlier on that same page in the Gemara text, we are told that Moses composed the Birkat hazan , that when the Manna fell, Joshua composed Birkat Ha’aretz when the people entered the Land of Israel, David and Solomon composed Boneh Yerushalayim at different stages of the building of the city, and the Rabbis at Yavneh composed HaTov ve’haMeitiv in response to the burial of the martyrs of Beitar!

Wherever they do come from the Birkat Hamazon, the blessings recited after food, is a different animal than most other blessings we do, which tend to be one-liners and which are focussed on the particular action involved and which, crucially, we do BEFORE the act. (The only one we do afterwards apart from Birkat Hamazon is the blessing after lighting the Shabbat candles, and then only so as not to say a blessing in vain, as we may bless and then not have time before Shabbat comes in to kindle the flame). Yet here in relation to food we bless BEFORE, with haMotzi, and then we go in in Birkat Hamazon to ask for blessing for the land, for Jerusalem, for God as protector as well as simply thank God for the food.
So why the extra Berachot, and why the need to extend the sense of our being blessed not only to the meal we have enjoyed but to the land of Israel, to Jerusalem, to our relationship with God? Nachmanides suggests that the long Birkat Hamazon is needed as an antidote to pride and arrogance. In the context of the biblical passage from where it comes, Moses warns the people against forgetting who is behind our food, who gives us our wealth – God. Moses warns that we must not just take our food from the land as if it is a right, but to remember the relationship we have with the land, that it is itself a symbol of our relationship of obligation to God.