Disk space used includes the contents of the home directory, e.g. website files, images, databases, any emails stored, any backups stored (webmasters should really store these elswhere), Logs, Caches, administrative files.
Over time Logs and Caches accumulate, taking up disk space.
Both folders can have their contents deleted to allow disk space and save from upgrading top the next hosting plan.
The Tmp folder contains web statistics. These are usually never referred to except for the current year, so can be deleted.
Emails are often left on the server because the user forgets to change the settings on their computer to delete them from the server. In that case these can be deleted to give more space.
Images have often been loaded straight from a camera. Their camera size may be 2 to 4 Mb but the web page may only need them at 100 Kb. WordPress optimizes the images before publishing each page but stores the original image at full size. These can be deleted to provide more space.