When Jesus taught his disciples to pray, He prayed "lead us not into temptation". Better than anyone, Jesus understood human nature and He knew that if you can prevent or overcome temptation you can avoid sin all together.

It's like praying, "Lord, don't just keep me from falling off the cliff, keep me away from the edge of the cliff all together!"  Jesus knew that his disciples, for all time, would be subject to the subtle processes and steps that gradually cause us to drift into compromise.  He taught us to pray for victory and deliverance before temptation ever matured into sin.  

It's a prayer for purity, consecration and devotion, not for those who want to toe the line and see how close they can get to sin without sinning.