I had no idea this was such an expected thing until people starting complaining about it.
If you're in a household where sound contamination (in or out) ever comes up, you know how magical game audio over a controller jack can be. Heck, even just people who want to play at night with an effortless, cheap solution for quiet-not-quiet gameplay. Or people who prefer stereo audio from headphones over normal TV speakers. It really has been pretty standard for years now.