Because regular downloads work like this: Thousands of customers download from a single server

It puts a lot of pressure towards that one poor server.
The uplink of this server is about 38 kB/s. Divide that by hundreds of customers, and you start seeing how "fast" you could download. I'd estimate about 0.1 kB/s. (And we're not yet talking about how long you would have to wait for the webserver to even answer your requests.)

Bittorrent works like this: Thousands of customers download from each others

It distributes the load. If the number of clients grows, so does the amount of upload capacity.
Download rates of 100 kB/s and greater are very well possible, because you download from all the fellows who are downloading the same file as you. Sometimes it's not that fast, because apparently not everyone wants to help the net to work. It's still faster than the "regular" way.

Trust me, this is the only option. Unless you want to buy me a 100 megabit internet connection.
Bittorrent is really cool. Your download rate is not limited by the server, because you're simultaneously downloading from all other clients of the same file.

But remember to return the favor - leave the download window open after your download is completed!
This is important for it to work!

If you are really helpful, read the chapter about seeding.