To begin to answer that question, let's look at how prayer has developed in Jewish tradition.
After the destruction of the Temple by the Babylonians in 586 BCE, the ancient Hebrews could no longer worship God by offering sacrifices. The Torah demands that the Temple is the only legitimate place for offering sacrifices to God, and so Judaism might have died along with the Temple. Instead, the ancient Hebrews kept their devotion to God alive by developing a new form of worship, one composed entirely of words of prayer. Later, with the destruction of the Second Temple in 70 CE, worship by sacrificial offerings ended for good and prayer was established as the primary spiritual practice of Judaism.
Since its invention, however, Jewish worship through prayer has gone through many changes and Jews have understood prayer in many different ways. The prayer service may have begun as a way to perpetuate the rites of the Temple and maintain a national/communal link with God, but in time the ancient rabbis turned prayer into a personal and spiritual act. The rabbis developed a philosophy of prayer that included the heartfelt qualities of intention and deepening a personal attachement to God.
The rabbis also made daily prayers part of their system of mitzvot—the sacred obligations of fulfilling God's will. So, performing prayers also became something that a Jew would do simply "because God told you to."
Later, kabbalah turned prayer into an act of mystical unification of the cosmos. By reciting daily prayers, a Jew would rise higher on the ladder toward ultimate attachment to God and effect restorative changes within the divine realm. Jewish mystical tradition transformed the meaning of prayer. To the kabbalists, prayer was not just a way to serve God; it was an act that turned the praying individual into God's partner in the repair of the world.
Of course, worship services are also social gatherings. By praying together, the community affirms its highest aspirations to its values and to God. Many Jews today put the experience of gathering together as a community as the most important reason for participating in prayer.
There is yet one more reason why Jews pray. Prayer is a tool of personal transformation. Like yoga and meditation, Jewish prayer can be viewed as a practice for personal development and honing a sense of personal equanimity, peace, self-awareness, and, ultimately, happiness. For many of today's Jews, this understanding of prayer is bringing a new sense of spirituality and meaning to their prayer practice.
What makes the most sense to you? Why do you pray? Or, if you don't, which vision of prayer would be most likely to draw you into the practice?