GoKawiilThe base Nintendo Switch Online plan, available for $20/year, includes all the essentials. Its main appeal is allowing you to play games online, either with friends or random people via matchmaking. You don't need Switch Online to play free-to-download games like Rocket League or Fortnite, but it's required for paid games (like Splatoon 3 or Mario Kart World). Thus, the value here depends on what you play.
Another core feature is the Nintendo Classics library, which includes around 200 NES, SNES, and Game Boy games. You'll find everything from masterpieces like Super Metroid and Super Mario Bros. 3 to more obscure titles like Game & Watch Gallery 3 and Crystalis. There's likely a lot in the library you've never tried before or would enjoy replaying, making this a great way to experience Nintendo's history. Some offer online multiplayer, but only with your friends.
Importantly, the subscription also provides cloud storage for your save data. That way, if your system ever stops working, all your progress is easy to move to a replacement. Given how long some games are, this is worth the price of admission alone.
Your subscription also grants access to Nintendo Music, a separate streaming app (and web tool) with various Nintendo soundtracks. This is neat, but not essential unless you're a huge fan of video game music. Finally, "special offers" include trials of certain games, missions that let you earn Nintendo Platinum Points and customize your profile's avatar, access to Tetris 99 and F-Zero 99, and the option to purchase retro replica controllers.