Well, when you are offering material for download, "Do I have the legal right to make this content available?" is a pretty legitimate question.
To put it in "How does following the law benefit me?" terms: It's not much use posting a link, if the said link is liable to be yanked three weeks later due to copyright claims. It's almost bound to make your own site look bad-- unless you're already so big that you can weather all inconveniences.