There is a very lengthy discussion on this topic here: Why do we need apps? It’s worth reading the entire thread.
My short answer to this is that my default is to always build my project as one app unless some condition exists to indicate otherwise.
For example, if I’m building a component to be used in multiple projects, then I want to structure it in such a way that it can be used as an app. If I know I’m going to be working on a Large system, my initial plans are likely to include multiple apps. (Large in my case typically refers to a system with more than 100 models or a projection of 10,000 lines of code.) But other than that, pretty much everything I build is built as a single app.