Super Smash Bros. Melee didn't ship with online features—it released in 2001 exclusively for the GameCube. The Dolphin emulator (opens in new tab) changed that, allowing players to compete online in what is still considered the best competitive Smash game.
Those online features are now greatly improved thanks to the development of Slippi (opens in new tab), a fan-made plug-in for Dolphin. It has an impressive features list, but most important is the implementation of rollback netcode, which is considered basically mandatory for competitive online fighters due to its efficiency at eliminating lag and input delay.
On the Switch, Super Smash Bros. Ultimate doesn't have rollback netcode, which now makes it far less viable for online competitive play compared to its much older (emulated) GameCube sibling. Some studios, like Mortal Kombat 11's NetherRealm, have retrofitted rollback netcode into already shipped games due to its superiority.
The netcode, especially, is a godsend for a competitive community forced to stay at home due to the COVID-19 pandemic. But Slippi offers a bunch of other cool stuff as well: It has online matchmaking for "quickly finding nearby opponents," it generates a wealth of gameplay statistics, and it automatically saves replays.
Smash pro Leffen outlines the benefits of Slippi in the video embedded below, and if you're wanting to install it yourself, there's also a tutorial here (opens in new tab).