This feature is kind of bandwidth throttling, ie. it allows you to limit the network bandwidth usage while uploading a backup file to its destination.
This feature is really useful because you want that your website visitors to have a pleasant experience while browsing your website. If a backup transfer is on progress and if it takes all the bandwidth while sending the data then your web visitors might notice a delay while they are navigating through your website. No one wants a angry visitor, it’s not good for the business, right?