Michiel,
WebTorrent Desktop behaves similar to other torrent clients. You need to manually add torrents to the client.
We don't want to make the client automatically download popular content, or seed it, or anything like that. The user should decide what to share since the content could be unencrypted and that's potentially dangerous.
If you're looking for some good torrents you can seed, I recommend the "Featured Torrents" at the bottom of this page:
https://webtorrent.io/desktop
Let me know if you have other questions. Cheers!
Feross