bandwidth means transfer of data and this is true the we can send a finite data in a single time through the line as processes in water's pipeline.
As well as if we consider the hard drives then it is also consist a limited space so that both a finite data transfer rate and limited disk space makes the thing impossible which is called unlimited bandwidth.
Some of few websites uses low space and not much bandwidth than Web hosting companies that provide unlimited bandwidth as a plan feature are cashing on the fact that you won't use very much things.
Most webhosting companies that offer unlimited as a plan feature will have restrictions on using the server resources, restrictions on upload and downloads of audio & video etc.Which generates problems so many times.
If we have lot of visitors on our site and if they are downloads songs,movies from our site than do not choose an unlimited plan hoping to find unrestricted bandwidth.