From the Forward to Winfield Bevins’ Our Common Prayer (paper, Kindle), this is Ashley Null:
Constant prayer, then, is the key to the Christian life. Of course, that is the whole point of the Parable of the Persistent Widow (Luke 18:1-8). We do not pray to God day and night because he is an unjust judge that needs to be prompted. We pray to him day and night because we need to be prompted. We struggle so much with injustice – the wrongs that others do to us, and the wrongs that we do to others. We pray to God day and night so that his love might renew a right spirit in us. We pray to God day and night for him to work in us so that we can forgive others their wrongs and give ourselves away in godly service. In short, we pray day and night, not to move the heart of God to want to do our will, but for God to continually move our hearts to want to do his will (16).
That about sums it up.