I figured I'd just clear up a small misconception. While Portal 2 specifically doesn't have a toggle, most other Source games do. Some people like being able to switch between the two as they please so they can still hear in-game audio while outside of the game (people with multi-monitor who play 2 games at once, for example). The audio settings template, for ease of use, tracks whether or not a game does that by default, but there are some games where there is a toggle.
Thanks for fixing my mistake on the Portal 2 page, I should have left it unknown if anything.