The Talmud in מסכת ברכות explains the reason why formal prayers were established. We pray three times a day during the week, and a fourth prayer, Mussaf, is added on Shabbat and holidays.
There are actually two reasons given, that overlap one another. The first is that the daily prayers were ordained by the Patriarchs.
The Torah hints that Avraham invented the morning prayer, Shacharit. When Yitzchak meditated in the field towards evening, Mincha was created. And before Yakov rested in the evening, he prayed the Ma’ariv or Arvit prayer.
The second reason in the Gemara for prayer, is that it takes the place of the sacrifices. Since we no longer have a Temple, we must regularly reach out to Hashem, in a formal manner. The Shacharit and Mincha services, correspond to the קרבן תמיד, the permanent sacrifice offered every morning and evening. There was an additional sacrifice, called, “Mussaf,” offered on Shabbat and holidays.
The overlapping of these two explanations is that there was no sacrifice at night. By the same token, there was no Patriarch that established Mussaf. Therefore, both explanations are needed.
The text of all these prayers were formulated by the Men of the Great Assembly, between the first and second Temple. Many of the members of these 120 scholars, were prophets, implying the great depth to the words we pray. It is our prayer that the 120 members of the new incoming Knesset, be granted similar Divine inspiration.