The Siddur is not merely a book containing the liturgy of the synagogue, but a comprehensive collection of prayers and benedictions relating to every aspect of Jewish life.
Not only may a person occasionally pray at home, just as he would in the synagogue, but there is an entire cycle of benedictions and prayers that are by their nature unrelated to the synagogue service–from the prayers recited on rising in the morning, through those that accompany a person throughout the day, until the recitation of the Shema upon retiring to bed.
Moreover, the order of prayers relates not only to the weekdays and festivals, but to the entire life cycle: from birth until death, upon reaching maturity and entering marriage, the Jew is accompanied by prayers and benedictions.
Nearly every sorrowful or joyous event, of everyday life or of special occasions, is reflected in the prayer book, since the Siddur is the book of Jewish life, in which everyone is involved and to which everyone relates.
But the Siddur's influence is derived not only from its use and from the connection to it but also from its contents.
No other Jewish book contains, as does the Siddur, the entirety of Judaism.
The Siddur is like a garland, intertwining all the strands of Judaism and encompassing all fields of Jewish creativity in all their variegated forms.
It includes sections that reflect the fundamentals of the Jewish faith, and those relating to the field of religious law.
There are some that describe a world outlook, and others that recount the central events in Jewish history.
The Siddur contains sections of exalted poetry, and matters of ritual procedure.
There are prayers that deal with the most intimate details of individual needs and problems, supplications reflecting the sorrows and aspirations of the entire nation, and prayers that touch upon the entire cosmos.
It contains texts based upon profound philosophical thought, and numerous others rooted in kabbalistic mysticism.
There are selections taken from the Torah, the Prophets and the Writings (Hagiographa), from the Mishnah and Talmud, from the Midrash, and kabbalistic works, from the stylized and complex poetry of medieval times, and from the private, personal prayers of Jewish sages.
In the same way as the Siddur represents the entire expanse of Judaism, it also reflects it through the dimension of time, in the generations of Jewish creativity.
Some passages, taken from the Torah and other books of the Bible, hearken back to the earliest history of the Jewish people.
Other formulations date to early Second Temple times, while there are prayers and benedictions composed during the mishnaic and talmudic eras.
It includes Piyyutim (liturgical poems) of the Middle Ages, sections composed long centuries ago, and prayer formulations created in our very own times.
–Rabbi Adin Steinsaltz
From A Guide to Jewish Prayer, p. 5, by Rabbi Adin Steinsaltz