An example of multiple calls to create would be if you had a title screen scene. When you’re game loads up, the player is showed that title screen so create is called then. Then you enter your game and the player then exits the game via a menu back to the title screen so create is called again the second time you enter the title screen.
The way Phaser manages its states is when you call this.state.add(…), this call will instantiate the state object (calling new MyState). It then store that instantiated object in a map using the key you provide to store it. Then when you call this.state.start(…), it pulls the state object from that map and calls init->preload->create.
EDIT: It’s also important to note that if you manually instantiate something in create and it isn’t added to the display lists in the scene, then you need to clean it up in shutdown. Because scenes aren’t nuked when you switch scene any object instantiated and not cleaned up will hang around until the scene is reentered and object is overwritten. It’s very easy to run into memory issues if you’re not aware of how the state system works.