The first way that this is useful is that you can point your DS to some fanmade server to get unofficial, fanmade event Pokémon. Indeed, this website exists for this very purpose. As of now, the website is currently distributing a Jigglypuff nicknamed Puffy, with the moves Hyper Voice, Protect, Thunderbolt, and Thunder Wave. It is at Level 41. All you need to get this Pokémon is a DS and Wi-Fi. The instructions to obtain it are on the website, however, I'll post it here anyway:
Then, as I said previously, in addition to the event Pokémon on that site, you can also use sendpkm for your own Pokémon. You can either create them from scratch with Pokésav, or if you already happen to be playing Pokémon on an emulator, you can take the save file and use Pokésav to create pkm files from your Pokémon. (Note: The Pokémon must be in your party, not a box!) You probably have to convert your emulator save file to a raw save file to do this -- use this online tool. And since the emulator Desmume supports Pal Park, it's even possible to get any Pokémon from emulated Ruby/Sapphire/Emerald/FireRed/LeafGreen to your real DS as well (which I've done before). So you can use sendpkm to get Pokémon from an emulator to a real DS!
As a gift for reading this whole thing, here are some Pokémon of mine for you to have, in the form of pkm files:
Deoxys (Defense Forme): http://www.mediafire.com/?dfnmljth3to
Mewtwo: http://www.mediafire.com/?nv50t3ybc2y
Kyogre: http://www.mediafire.com/?nr5ymgadmnm
The Deoxys and Mewtwo were obtained from an emulated LeafGreen, and the Kyogre was obtained from an emulated Sapphire. Deoxys has a Calm (+SD -Atk) nature, Mewtwo is Timid (+Spd -Atk), and Kyogre is Modest (+SpA -Atk). They all also have rare berries: Deoxys has Liechi Berry (a very elusive berry -- as far as I know, it's only possible to get in Mirage Island, event Shiny Zigzagoon, and the recent event Jirachi), Mewtwo has Salac Berry (which can be obtained from Colosseum among other ways), and Kyogre has Petaya Berry (also obtainable in Colosseum). To be honest, I cheated to get the berries, but I don't think anybody will mind. They were all freshly caught, meaning that none of them gained any Exp., or have any EVs, and their movesets are whatever they were when I caught them. I don't know what their IVs are, because I didn't bother to check.
(Wondering why these Pokémon seem to gain a lot of experience, even for traded Pokémon? Here's a hint: check their Pokédex entries if you have Platinum, HeartGold, or SoulSilver.)
Finally, I have more Pokémon coming soon... I can't say what they are going to be though, except I can't promise ANYTHING from Generation IV.