5th Gen Game Sync

Back in generation V, you could tuck a Pokemon into game sync and enter the Dream World. You also would receive stuff through it, such as C-Gear skins, Pokedex skins, and Musicals. Game Sync also existed in Generation VI and VII, but all it does is update your data and synchronize stuff.

Did you make use of game sync? In the later two games, I believe it's mandatory whenever you go online, but I did make use of it in Gen V voluntarily.

When Black came out I definitely did, mostly for the Dream World abilities, and berries - I didn't, and still don't, care about the Pokémon Musical element of the game. When you could only play it for 1 hour every 20 hours given my terrible internet connection I couldn't get much done. I didn't get Black 2 until 2015, and no online functionality was available.
It was a neat feature at the time, but as someone who has never, and will never, buy large amounts of games it frustrates me how such a significant feature was separate to the game and thus only supported for a few years. In Gen IV at least it was only trading and battling - whilst I can't battle myself, due to luckily having multiple DS systems and games I can trade with myself for most things, but with Gen V the removal of berries mean I can't EV train my team, something I really enjoy doing, or even just have them hold berries outside of Battle Subway (as someone who started out in Gen III, berries are a big thing for me). Don't even get me started on the fact that they programmed a shiny Haxorus into B2W2 that I have no way of accessing...