The tracker url is included in a magnet link. Users who click on that link will get the IPs of others who clicked it, so they do know sources to download the linked to file from.

__________________
</td><td><a href="http://crawler.trillinux.org"><img align="right" style="border:0" src="http://crawler.trillinux.org/img/netsize.png" alt="Network size"></a></td></table>