There are some interesting verses in Psalms 115:16 and Phil 2:13 that help explain why God works through praying men when He wants to do something in the earth, rather than doing it all Himself. These verses declare:
Psa 115:16. "The heavens are the heavens of the LORD, But the earth He has given to the sons of men."
Phil 2:13, "For it is God who is at work in you, both to will and to work for His good pleasure."
Prayer is not the means whereby we get God to do what we want Him to do. Prayer is intimacy with God whereby He changes our heart and brings us into His heart. As we pray His heart to Him, and He does His work in the earth through us for His good pleasure.
This form of prayer is quite a bit different than our usual self-centered petitions.
Blessings