Step out of technical thinking for a minute. Think you in your house. To you more doors are open and are accessible because you are inside the house, so you have a much different viewpoint, not to mention you have the keys to everything.

Now, if you're a would-be burglar scouting the neighborhood, you only see things from the outside, and hopefully you don't have the same keys. What doors are "open" to the guy inside might be "closed" to the guy outside. So, if you're checking the security of your house, you begin from the outside.

Such is the same with how you should scan your network for vulnerabilities.