R.C. Sproul has a two part series on the power of prayer over at the Ligonier Ministries Blog.
these answer further the question of “why pray?” In Part 1, Mr. Sproul uses a Hebrews 11 format to list some things that were accomplished through prayer in the Bible.
By prayer, Esau’s heart was changed toward Jacob, so that they met in a friendly, rather than hostile, manner (Genesis 32).
By the prayer of Moses, God brought the plagues upon Egypt and then removed them again (Exodus 7-11).
By prayer, Joshua made the sun stand still (Joshua 10).
By prayer, when Samson was ready to perish with thirst, God brought water out of a hollow place for his sustenance (Judges 15).
By prayer, the strength of Samson was restored. And he pulled down the temple of Dagon on the Philistines, so that those whom he killed as he died were more than all he had killed in his life prior to that (Judges 16).
By prayer, Elijah held back the rains for three and a half years. And then by prayer, caused it to rain again (1 Kings 17-18).
By the prayer of Hezekiah, God sent an angel and killed in one night 185,000 men in Sennacherib’s army (2 Kings 19).
By the prayer of Asa, God confounded the army of Zerah (2 Chronicles 14).
And time would fail me to tell of Abraham, who prayed for and received a son at the age of one hundred years; and Moses, who received help at the Red Sea; and the Israelites, who were delivered from Egypt after much prayer; and David, who escaped the treachery of Saul by prayer; and Solomon, who received great wisdom as the result of prayer; and Daniel, who was able to interpret dreams after prayer. People were delivered from peril, healed from diseases, saw loved ones cured, and witnessed innumerable miracles as the result of fervent prayer.
why pray? see the list above.