Office Addins, why do you need to host an app?

I was initially pretty keen on hearing about using Office apis with JavaScript but I'm pretty confused; Why do they need to be hosted? I could see this being interesting if you had some external machine learning or other resource intensive application that you wanted to run in the cloud. But for apps not of this nature, and even simple extensions, it seems over kill.

Doesn't this mean that you always have to be online for any of your Add-ins to work?

I guess I'm not seeing a huge difference between embedding a webpage in an excel file or using nodejs to use an api.