Minecraft wiki entry for NBTExplorer, associated text: Locate in-game structures;
Add or change items, and adding enchantments;
Access to the world seed;
Changing gamerules without usage of the /gamerule command or new gamerule changer built in when creating a world;
Applying custom formatting to world or server names;
Change the world's name;
Change the players game mode, health, position and so on. For example, if the player is in an "illegal area" and can't teleport back with /tp, this function can change the player's position.
Change the settings for terrain generation, while it doesn't affect the generated chunks;