You have got the concept of roles of torrent files and magnet URI wrong. A couple of Wikipedia quotes for you.
To share a file or group of files, a peer first creates a small file
called a "torrent" (e.g. MyFile.torrent). This file contains metadata
(e.g. video, audio, image, application, text) about the files to be
shared and about the tracker, the computer that coordinates the file
distribution. Peers that want to download the file must first obtain a
torrent file for it and connect to the specified tracker, which tells
them from which other peers to download the pieces of the file.
The Magnet URI scheme is a de facto standard (instead of an open
standard) defining a URI scheme for Magnet links, which mainly refer
to resources available for download via peer-to-peer networks. Such a
link typically identifies a file not by location, but by content—more
precisely, by the content's cryptographic hash value.
Basically, the torrent files includes all the necessary info about the shared files to download via the BitTorrent protocol. You can't download any files shared via torrents without these metadata. Magnet URI on other hand does contain any data within, simply encoding a name for the application to seek from peers, in this case the torrent in question. So if you have a magnet URI and no peer with the torrent it refers to, you will never get the files it shares. You will notice (specially if you have a slow connection) that μTorrent mentions "downloading metadata" when you open a Magnet URI. That is downloading the torrent itself from the peers already holding it. If you don't find any peers, you won't even reach this step.