Why do torrents upload when you are downloading?

First thing you should know is that if your upload speed is high, it drastically increases your download speed as well.

When you are downloading/uploading the file, you are referred to as the 'leecher'. Once your download is complete, you will see that your Bittorent client will keep on uploading the file, since downloading takes much less time than uploading. Once you have completely uploaded the file, you are known as a 'seeder'. The more the seeders on a torrent, the faster the downloads. That's why its called P2P (peer-to-peer), so you download and share at the same time!