Why would I use a Sprite Atlas?

I just looked into the new Sprite Atlas system and am a little worried. This seems like its meant to replace the old one (that is getting labelled as legacy) but it doesn't function in any way as well as the original. Granted the original has issues, and the Folder based filling is really nice in Sprite Atlas's, but you have to write code to reference these textures. This must surely be an occasionally used case and would surely not replace the original system?

Or am I missing something? I actually think I am so that's why I'm asking!

 

Anyone has more information on how Sprite Atlas works ?

The manual ( https://docs.unity3d.com/Manual/SpriteAtlas.html ) don't give much insight on how it works behind the scenes.