@seedhe - The module framework that @MichMich created is where the technique of hosting the mirror on a Pi really shines. While the Magic Mirror is more-or-less a customizable web page, the ability for the modules to call background processes adds a level of utility.
For example, I can put a static web page up on an Amazon cloud IP and just hit that from my mirror. It will display static data and maybe some things from various APIs, if I code it cleverly enough. But that’s about it.
With the OS working behind the scenes, I can display anything behind my firewall (IP cameras, weather stations, temperature sensors, PIR sensors, etc.). I can also plug in a camera and/or microphone and use the data from those devices in my modules.
It also allows you to scale up if you need to. A Raspi is pretty slick for its size and cost. But if you put something like a NUC behind it, you can use that processing power for some really cool features. Voice recognition, facial recognition, complex animations, etc.