The Asset Pack file is a manifest (in JSON format) of the files you can load in a game. It is built-in in Phaser and you can use it without Phaser Editor.
Learn more about Asset Pack loading in Phaser
This file is intensively used in the IDE. Tools like the Scene Editor depends on the information provided in the asset pack files.
You can have multiple asset pack files in the project. In fact, it is a good practice in large games to split the assets in different groups (asset pack files) and load them only when it is required.