Shell commands are used when you want to access relatively inaccessible locations in Windows. They can also be used to create desktop shortcuts.

Windows 7 has a few new shell commands.

Some of the useful ones are:

shell:CommonRingtones
shell:Device Metadata Store
shell:DocumentsLibrary
shell:ImplicitAppShortcuts
shell:Libraries
shell:MusicLibrary
shell:OtherUsersFolder
shell:PublicSuggestedLocations
shell:User Pinned
shell:PicturesLibrary
shell:Ringtones
shell:UsersLibrariesFolder
shell:VideosLibrary

Source: TheWindowsClub