Directories are a multipurpose tool that you can utilize to store and use different types of information. You can determine which information you need to store in your Planfix account, and you can use directories to configure how it's stored.
Account administrators can create, edit, and delete directories. Other employees can view directories and add or edit individual directory elements (entries) if they have access to the directory.
Read more about that here: How do I add information from a directory to a comment?
In addition to directories created by account administrators, there are also Planfix-specific directories, which cannot be deleted. They are located in the section Account management / System directories.