The Pokemon Games - We split this into a separate category because the GBA games are the main product in this case.Note that Sonic Adventure 2: Battle can also connect to a GBA without a game for a slightly lesser version of what it does. Many games do this, including Harvest Moon: A Wonderful Life, Sonic Adventure 2: Battle, and Metroid Prime. Connectivity with a GBA game - Titles could also connect with specific GBA games to unlock content or do things between one or both games.This may seem like a rather expensive bonus feature, but at least it's not an amiibo to unlock quick warping, right? Animal Crossing's GBA island, Mega Man X: Command Mission's Treasure Tracker and Wind Waker's Tingle Tuner are great examples of this. Bonus Modes or Extra Content - Games could also use the GBA in a single player context to unlock bonus content on the main game.
Examples of games with this feature include Nintendo Puzzle Collection, Animal Crossing, and technically Crash Bandicoot: Wrath of Cortex. Sometimes it is a bonus NES game, sometimes it's a miniature version of the game you're playing.