DirectoryExists service checks whether a certain directory exists. The service will look for the path on application server, unless you specify the target server name or IP address in the directory path.
This can be helpful when you need to dynamically create a directory based on the user or entity the files you wish to save are associated with. For example, saving images or message attachments that are stored in a directory that correspond to the particular message/userid it belongs to. You will first need to check if the directory exist and if not, create it.