When running a backup there often are some files and folders that you do not want to include in the backup. In creating a disk image, there are usually temporary or unimportant files you don’t care about. Examples include the memory swap file, hibernation file, recycle bin, temporary OS (and program) files and folders. If you are running a more frequent file based backup of your personal files, you may want to exclude these also.
Many backup tools offer the ability to exclude files and folders using complete names (e.g. swapfile.sys) and wildcards (e.g. *.tmp), often automatically populating a list of commonly excluded files as a starting point. Using this feature will save some disk space and data transfer time.