Its advantage over plain http is that when multiple downloads of the same file happen concurrently, the downloaders upload to each other, making it possible for the file source to support very large numbers of downloaders with only a modest increase in its load.
BitTorrent is a simple software product which addresses all of these problems.
The key to scaleable and robust distribution is to tap the unutilized upstream broadband capacity of your customers. It's free and abundant. Their contribution grows at the same rate as their demand, creating limitless scalability for a fixed cost.
BitTorrent is not just a concept, but has a functioning implementation, already capable of swarming downloads across unreliable networks. BitTorrent has been embraced by numerous publishers to cost-effectively distribute to millions of users.
BitTorrent is the result of over two years of intensive development.