I believe it's to verify that you legitimately purchased the game. But, everyone is right in that having the online connection severely limits how much of the game is available to you, which does affect the enjoyment of the game. Sure, you can play offline, it is possible. However, additional content you receive from missions such as weapon and gear will not be available to you. Contracts mode and challenges are also locked out for you, and challenges are the only way to unlock additional gear, because you need points for "Level Mastery". Essentially, offline mode leaves you with a base game where the only basic gear is available to you. Additional starting disguises, starting mission locations and smuggle points are also locked out for you. Even if you unlocked that content while you were online and decide to play offline the next time you boot it up, that content will be locked out to you. Not only this, but having the constant connection to online will also slow down the game. Loading the game and loading previous saves used to be way too long. This has been patched to be a bit shorter, but unfortunately it's still noticeable that the load times are a tad on the long side. The game even knows what saves were done "Offline" and which ones were done "Online", so you technically have two types of saves.