It was instituted to recite a blessing over any benefit that we enjoy in order to thank God for the good things that He has given us. This type of brachos are called “birchos hane’enin,” blessings over things from which we derive benefit. Foremost among these are blessings over food, such as reciting hamotzi over bread or ho’eitz over fruit, though there are also blessings recited over pleasant fragrances. The concept is that if one derives benefit from this world without thanking God, it’s considered misappropriation. (It’s tantamount to walking into someone’s house and just taking food without even asking.)
Blessings are also recited after eating and drinking and before performing many mitzvos. This is true even for the Rabbinic Mitzvos, such as those we are currently discussing. However, as we said in our introduction, there is one Rabbinic Mitzvah over which a blessing is not said: that would be this mitzvah, since one does not say a blessing on a blessing. (Birkas HaMazon, AKA “bentching,” is a Biblical mitzvah. Even so, we do not recite a blessing that God commanded us to recite grace after meals.)
The final category of brachos are those that praise God, such as for His works of nature. King David instituted that a person should recite 100 blessings every day.
This mitzvah is discussed, appropriately enough, in tractate Brachos, specifically in the sixth chapter, “Keitzad Mevarchin” (“how do we bless?”), which starts on page 35a. The obligation to recite brachos is codified in the Shulchan Aruch in Orach Chaim 202.