Here's why:
* Timeline: Avatar: The Last Airbender takes place 70 years before The Legend of Korra.
* Characters: Korra, the protagonist of The Legend of Korra, is inspired by Avatar Aang, the protagonist of Avatar: The Last Airbender.
* World Building: Both series share the same world, culture, and history, with Avatar: The Last Airbender laying the groundwork for the events in The Legend of Korra.
So, while both are great shows in their own right, it's best to watch Avatar: The Last Airbender first to understand the full context of The Legend of Korra.