It is a positive mitzvah from the Torah to bless [God] after eating satisfying food, as [Deuteronomy 8:10] states: "When you have eaten and are satiated, you shall bless God, your Lord."
The Torah requires a person to recite grace only when he eats to the point of satiation, as implied by the above verse, "When you have eaten and are satiated, you shall bless...." The Sages, however, ordained that one should recite grace after eating [an amount of bread equal] to the size of an olive (approximately 1 oz).
Similarly, the Rabbis ordained that we recite blessings before partaking of any food even the slightest amount of food or drink and then derive benefit from it.
Similarly, when smelling a pleasant fragrance, one should recite a blessing and then smell. Anyone who derives benefit [from this world] without reciting a blessing is considered as if he misappropriated a sacred article.
The Rabbis also ordained that one should recite a blessing after eating or drinking, provided one drinks a revi'it and eats a k'zayit. A person who [merely] tastes food is not required to recite a blessing before partaking of it or afterwards unless he partakes of a revi'it.
Just as we recite blessings for benefit which we derive from the world, we should also recite blessings for each mitzvah before we fulfill it.
Similarly, the Sages instituted many blessings as expressions of praise and thanks to God and as a means of petition, so that we will always remember the Creator, even though we have not received any benefit or performed a mitzvah.
All the blessings can be divided into three categories:
blessings over benefit;
blessings over mitzvot;
blessings recited as expressions of praise and thanks to God and as a means of petition, so that we will always remember the Creator and fear Him.
The text of all the blessings was ordained by Ezra and his court. It is not fit to alter it in any way. Whoever alters the text of a blessing from that ordained by the Sages is making an error.