For me, tasks that go in the default task note are mostly individual actions that I just need a reminder for. They have little context. Eg: 'Book appointment with dentist'
Like @gazumped it is also the place to capture an action that doesn't, yet, have any context. Once I have context I move the task to that note or, very occasionally, I add a note link to the task in the default task note.
For me, the real power of Evernote tasks is the ability to create and connect context. So a note with content that contains the connected tasks can also be connected to the calendar. I think it was @Stacey Harmon who helped to recognise that notes are the starting place for both tasks and calendar.