This is a very good question. In general, you should use a Directory server when you require data to be centrally managed, stored and accessible via standards based methods.
Some common examples found throughout the industry are, but not limited to:
- Machine Authentication
- User Authentication
- User/System Groups
- Address book
- Organization Representation
- Asset Tracking
- Telephony Information Store
- User resource management
- E-mail address lookups
- Application Configuration store
- PBX Configuration store
There are various Distributed Schema Files that are standards based, but you can always create your own Schema Specification.
There are always new ways to use a Directory and apply LDAP principles to address certain problems, therefore there is no simple answer to this question.
If in doubt, join the general LDAP forum for non-commercial discussions and information relating to LDAP at: https://www.umich.edu/~dirsvcs/ldap/mailinglist.html and ask