Why and why change scenes in UNITY?

Question

Unity

Why scenes change in UNITY is taught in all lessons and there is no description of this.

I use to distinguish between the current position of the user in the main menu he or in the game. But this can also be done through variables.

I'm assuming this is needed:

• To optimize each scene (main menu, game scene, locations)
  
• To determine the position (main menu, game scene, locations)
  

But then I don't understand. Should scene transitions be used for mobile casual games with simple mechanics? Is this something recommended?