Doing this means the torrent app will only download torrents if it’s connected to the VPN’s IP address. That way, your P2P traffic doesn’t leak if the VPN disconnects while you’re using the torrent client.
While that’s useful, I don’t think it’s 100% necessary. As long as you’re using a VPN with a good kill switch, you don’t need to bind the VPN’s IP address to the torrent client — if the VPN disconnects, the kill switch will disable all online access, preventing you from torrenting without the VPN.