Why We Pray

The Talmud in Masechet Berachot teaches the concept that תפילה במקום קרבן, that prayers take the place of sacrifices. Just as there is a commandment to offer the daily “Tamid” sacrifice morning and evening, so must we pray every morning and evening.
The Tamid was there to remind us of our priorities. Our first thought when we begin each day, is to serve Hashem, by praying to him. Similarly, as the day comes to a close, we also acknowledge our complete dependence upon Him. Every day of the year, two sheep that were one year old were offered in the Temple, one in the morning and one in the evening. This became our anchor as we dealt with our daily activities. We offer a public sacrifice to Hashem, to affirm that He is running the world.
On Shabbat and festivals, there is an an additional sacrifice known as the “Mussaf” sacrifice. This is why we have an additional prayer on these special days, called the Mussaf prayer.
Prayers took on a greater role of importance, once the Temple was destroyed. The Men of the Great Assembly addressed this issue even before the second Temple was built. The majority of the Jewish people were living outside of Israel during the Second Temple period.
A formal Siddur and prayer service was established to satisfy the need to pray in place of sacrifices. This was one of the great contributions of the Men of the Great Assembly. This helped keep the Jewish people connected no matter how far away they were geographically from Eretz Yisrael.