In simple words, Bandwidth can be defined as amount of data transferred from host server to other devices. Bandwidth is measured in similar quantity for space i.e is GB.
For example, Lets say, One website has page size of 1.4 MB and it is completely loaded on two different device, so total bandwidth will 2.8MB.
It is irreversible, It can’t be reversed. Crawlers also use some bandwidth for extraction of data. It is also good measure to calculate traffic on website.
Deletion of files on server doesn’t reduce bandwidth but reducing the page size can help admin to save the bandwidth of the server.