Bandwidth is a resource service providers pay for. If they are in business to make money, service providers must find a way to pay for their expenses, and make sure that there is some profit left over. Bandwidth is data transfer rate.
More visitors mean more data transfer.
With large enough number of visitors initiating more and more data transfers, the demand for more files will exceed the service provider's maximum data transfer rate. Promising unlimited bandwidth is misleading and possibly fraudulent. Responsible service providers never promise unlimited bandwidth. Service providers who meter bandwidth and charge according to bandwidth consumption have an incentive to make sure that their customers' web sites are always available to visitors.