You find a library on the Internet somewhere. What entitles you to copy it onto your machines? Copyright does not allow to do this, so you are reliant on a license to give you permission to do this. GPL gives you permission to do this, as long as when you distribute any modifications of it, you also pass along the source code. AGPL gives you permission to do this, as long as you give the source code to all of your users should they request it. If you can't meet the conditions, than you don't get the rights acquired by the license, and you fall back to default copyright, which says it's illegal to have copied the library to your own machines in the first place.