Do we pray to avoid doing other work for God?
No, we pray because our Lord prayed and, so, urged us to pray by example.
We pray because the various persons noted in the Old and New Testaments all engage in prayer.
We pray because we see how the various writers of the epistles in the New Testament urge us to pray.
We pray because when God is shown to work in miraculous fashion our faith is confirmed and God gets the glory. Prayer, then, is the primary work of the disciple and the means by which all our other works find their place and power.