The high-quality scans, provided by your lab, take up a great deal of space on the server. The lab may have a policy on how long your files may remain. Still, after everything is safely downloaded, and perhaps backed-up, it is recommended to be a good guest, and delete the server files yourself.
The deletion buttons in the toolbar work similarly to the ones for downloading. Note that deletion may only be done on entire rolls, not individual files. The deletion functions are locked out while any downloads are in progress.