From the Pulpit...

This is somewhat of a unique parable, because the lesson is given plainly and up front: people have an obligation to pray, and not to give up. That obligation is set upon the certainty that our God is far better than any unjust judge and will hear and answer believing prayer.

The prior verses, in chapter 17, speak about the second coming of Jesus, the Son of Man. There will be a time of sudden judgment like in the days of Noah and Lot, in which people will be taken away in judgment. Jesus' words about prayer are colored by this context.