Java heap max size was increased to 1200MB (and later 2GB for 64-bit clients) to improve performance and prevent pauses during garbage collection. Loot Respawn:
If you load up Build 39 today, you will immediately notice several jarring differences from the modern game. These features are what make the build "exclusive."
Build 39 is historically significant because it was the final stable release before The Indie Stone unleashed the monumental Build 41. This makes Build 39 the final iteration of the original engine’s combat and movement systems.