Core academic services like e-mail and the web require only a tiny fraction of our Internet bandwidth at any one time. There might be an occasional burst of data, when someone downloads a video file, for example, but in general most time online is actually idle time as far as the network is concerned. This is what makes it possible for the entire campus to share Internet bandwidth.
The problems start when you throw high demand, automated applications like peer-to-peer file sharing into the mix. Such programs are capable of consuming huge quantities of bandwidth with little or no user interaction required.
In a typical case, an individual will queue up a bunch of downloads and then walk away, leaving the machine to "do its thing" while they eat lunch or attend class. After those downloads finish, the software adds those files to its list of available downloads and serves them back to anyone who submits a request. Even if that user never downloads another file, the existing files will continue to be served to all comers until the software is disabled or uninstalled. It's not uncommon for a single computer to be running 20 or more simultaneous uploads and downloads, all without the owner's knowledge.
Experience has shown that unattended, high-demand network applications like peer-to-peer file sharing can, if unchecked, render our Internet link completely unusable. Traffic studies over the years confirm this.