Awesome to hear you’ve got your head around it.
In my mind there isn’t any one reason you’d use a single page app over a multi page app. It really comes down to the type of user experience you want to create and what the team working on the project is comfortable with. So if you want to create a really dynamic web app with lots of user interaction, holding state (thats a big one) and provide the best user experience possible then a single page app is going to serve you well. However there are a number of downsides that come along with building a SPA. Namely you now have 2 code bases and 2 apps instead of one to look after, slower initial load times, SEO issues, javascript (10 + [] = "10" … obviously).
So choosing when to use one over the other is really a per project thing. For instance at Testing Pays we have an ember app for our main site . But our “front page” site (the marketing one) is server redered html, we’re looking to add some functionality to that one soon and we will likely use glimmer for that.
Those are my quick thoughts on it, hope it helps 
