Downloading Ubuntu ISO images with a torrent client is one of the most efficient ways to obtain the installation media. Torrenting allows you download the file from multiple sources simultaneously, which means you’ll wind up with a very speedy download because the entirety of your network’s download bandwidth can remain saturated. It’s a very reliable method for downloading large files.
It also takes some strain off of official Ubuntu mirrors, so you’re actually helping them out a little by getting an ISO image from the torrent swarm. If you want to continue helping after the download completes, you can choose to leave the Ubuntu torrent in your torrent client and let it “seed” (torrenting jargon for upload) to other prospective downloaders.