As a Calvinist I am often asked questions like, 'why pray? if God has made up his mind regarding how he will act (what he will ordain), why pray?'. Or, "why bother doing missions or evangelize? if God has already predestined people, why do we need to put forth the effort?' The best answer to these questions is that God, our Creator King, has commanded us to do so. He commands, we obey. A more full answer includes a discussion of God using means to accomplish his ordained purposes. He ordains the ends to be accomplished as a result of means (which he has also ordained). So God heals an ill saint or brings one of his children into the kingdom in response to the prayers of his people (and the witness of his people), which he has commanded and ordained.
You see a great example of this in Job 42:
"Now therefore take seven bulls and seven rams and go to my servant Job and offer up a burnt offering for yourselves. And my servant Job shall pray for you, for I will accept his prayer not to deal with you according to your folly. For you have not spoken of me what is right, as my servant Job has" (Job 42:8).
God has promised that he will accept Job's prayer for his friends and relent from sending judgment for their folly. He has commanded Job to pray from them, and has determined already how he will respond. Does this free Job from the responsibility of actually praying for them? Is he off the hook? Can he say, "God's said he's gonna forgive them, why does he need me to ask?" No. Job, we can assume, obeys God and his friends are forgiven. Ordained end in response to a commanded and ordained mean.