The /data command lets the user change any NBT data of any entity - except for the player. This is done for a good reason. And while this prevents people from corrupting their save file, it also limits map makers with what they can and cannot do. This is where this mod comes in. The game no longer checks* if the modified entity is a player, allowing for things like:
-Easier player inventory modification
-Immortal survival players
-Mortal creative players
-Players with no gravity
-Freezing players
-Easier player inventory modification
-Storing more complex data right in the player, instead of relying on scoreboards
-And more!
It should however be noted, that while you can modify some of the NBT data you couldnt before, it wouldnt actually do anything. For example, giving the player CustomName data wont actually change their displayed username. And although some of this functionality might be added later, Ultimately the goal of this mod is to just remove the player check, nothing more.
(Also, if you manage to severely break the game, please leave a comment, I would love to see it lol)