To have an overall thing. If you say 'we're playing in north america' that tells us something. If we say europe, or asia, that gives us soemthing quite different. The 3I is a continent. The sectors are countries within that continent (ok the analogy isn't super accurate, but it should get the point across).
We CAN say that every country, or state, or city, on the continent is defined. But it would also be completely reasonable to say 'that place between france and germany, has a country or more than one, but we aren't going to define what those countries are'.
But by defining france and germany, as well as the overall europe, well, we still have a whole lot of information about what those countries might be like. And most people could just set their game in france or germany. But if you want to be in europe, and you have a cool idea for a country, well, you have a spot that isn't defined - specifically so that players don't say 'but, the rulebook says the country is like this!'
Having an empty spot, avoids a lot of arguments for many tables, whether that's necessary for your table or not.