1.When you want to apply the query to all of the files and all of the subfolders in the current folder
2.When you want to apply the query only to the files in the current folder and not to any of the subfolders
3.When you want to apply the query to all of the files in all of the folders and not to any of the subfolders in the entire site
4.When you want to apply the query to all of the files in all of the folders and all of the subfolders in the entire site