Above: A futuristic hot air balloon beams Pokemon back and forth between your DS and Pokemon Ranch

You can't level up your Pokemon while they're in the ranch
From a training perspective, there's no real benefit to sending your Pokemon to the ranch. You can't battle, level up, teach new moves, or otherwise train your Pokemon while they're uploaded to the ranch. Think of it as storage for all the Pokemon you're no longer using, like a retirement home for Pokemon stagnating on your PC.