If God has already willed to send rain, then why pray for it? Or if God has chosen to save you, then why fight so hard against temptation?
Edwards gives his answer to questions like these in Miscellanies #29 (reformatted for readability):
God decrees all things harmoniously and in excellent order; one decree harmonizes with another, and there is such a relation between all the decrees as makes the most excellent order. Thus God decrees rain in drought because he decrees the earnest prayers of his people; or thus, he decrees the prayers of his people because he decrees rain.
I acknowledge, to say God decrees a thing "because," is an improper way of speaking, but not more improper than all our other ways of speaking about God. God decrees the latter event because of the former, no more than he decrees the former because of the latter.
But this is what we [mean]: when God decrees to give the blessing of rain, he decrees the prayers of his people; and when he decrees the prayers of his people, he very commonly decrees rain; and thereby there is an harmony between these two decrees, of rain and the prayers of God's people.
- when he decrees diligence and industry, he decrees riches and prosperity;
- when he decrees prudence, he often decrees success;
- when he decrees striving, then often he decrees the obtaining of the kingdom of heaven;
- when he decrees the preaching of the gospel, then he decrees the bringing home of souls to Christ;
- when he decrees good natural faculties, diligence and good advantages, then he decrees learning;
- when he decrees summer, then he decrees the growing of plants.
Thus, when he decrees conformity to his Son, he decrees calling; and when he decrees calling, he decrees justification; and when he decrees justification, he decrees everlasting glory.
Thus all the decrees of God are harmonious; and this is all that can be said for or against absolute or conditional decrees. But this I say, it's improper to make one decree a condition of another, any more than the other a condition of that; but there is a harmony between both.