The essence of prayer–man's turning to his Creator in supplication and gratitude, with praise and petition–is a quintessential part of man's relationship with God, and as old as man's existence on earth.
This elemental form of prayer, however, has no fixed pattern.
Rather, it springs from the innermost emotions of the human heart, or from the social cohesion that occurs when a group of individuals join together as a congregation to express their sense of faith and devotion to God.
Even in the Torah, whose central concern is the creation of frameworks and pathways by which the Jew can relate to God, there is no fixed or sharply defined structural pattern prescribed for prayer.
Some sages hold that the Torah obligates a person to turn to God daily to offer Him this "service of the heart," while others think that the Torah requires a person to pray only in times of distress.
All concur that the biblical commandment of prayer does not impose specific times for worship, and certainly not any fixed formulation.
However, since earliest times there have been individuals who set aside regular periods of the day for offering God this service of the heart, which the Zohar (Leviticus 68a.) calls "the Service of Love."
In the words of King David, "Evening and morning and noon will I pray and cry aloud, and He shall hear my voice" (Psalm 55:17).
Later, we find fixed times for prayer recounted in Daniel, "His windows being open in his chamber towards Jerusalem, he kneeled upon his knees three times a day, and prayed and gave thanks before his God" (Daniel 6:10).
But the maintaining of fixed hours for worship was the custom of only a few individuals, who felt an inner need to address themselves to their Creator at regular intervals.
The majority of the people prayed whenever the need arose, whether in response to their inner emotions, or in times of distress, or when they had some special petition to make to God.
At the beginning of the Second Temple period, this situation was radically altered by the Sages of the Great Assembly, the leaders of the people at that time.
As the result of exile and oppression, a large part of the Jewish people in both the Land of Israel and in other countries had lost the sense of continuity with their heritage.
Familiar traditions that had been observed during the First Temple period were forgotten or became confused, while even the Hebrew language became adulterated with foreign words and idioms.
People found it difficult to express themselves in any form of speech, and particularly so in their own language, the Holy Tongue.
In addition to the immense effort, undertaken by the religious leaders of Israel-beginning with Ezra the Scribe–of reeducating the people, it was necessary to set down clear and specific guidelines that might be learned and transmitted from one generation to another.
–Rabbi Adin Steinsaltz
From A Guide to Jewish Prayer by Rabbi Adin Steinsaltz