Why do we pray?

Then Jesus told his disciples a parable to show them that they should always pray and not give up … And will not God bring about justice for his chosen ones, who cry out to him DAY & NIGHT? … I tell you, he will see that they get justice, and quickly. However, when the Son of Man comes, will he find faith on the earth?”    Luke 18:1,7,8 NIV (excerpts)