Do somebody/something the world of good
Definition 1 of 1
Pro Tip 1/3Positive Change
Used when something results in a very positive or healthy change for a person or situation. 
After studying harder, his grades did him the world of good.
Pro Tip 2/3Figurative Expression
It's a figurative way to say something has a very good effect, not that the whole world is literally involved. 
Her kind words did me the world of good during a tough time.
Pro Tip 3/3Contextual Use
Often used when the improvement is observable by others or is emotionally significant. 
Seeing his grandchildren did the old man the world of good.