What are the main strengths of LDAP and LDAP directory servers?

They are optimised for authentication, authorisation and address-book lookups; they read very fast thanks to denormalised storage; they replicate easily to distributed sites; and LDAP is the de-facto industry standard that almost every application supports.

• Built for IAM lookups: password checks, permission checks via groups and address-book searches are exactly the queries the protocol and servers are tuned for.
• Fast reads: an entry keeps all its attributes together (not normalised into several tables), so one lookup returns everything about an object.
• Distributed data: redundant local copies at distributed locations, loosely coupled replication between them, and very high availability without complex configuration or high cost.
• Broad application support: VPNs, mail clients, operating systems, web servers and countless business applications can talk LDAP out of the box.

The flip side of the same design: no transactions and no joins, so a directory is the wrong place for fast-changing business data such as orders or session state.

Go deeper:

• OpenLDAP Admin Guide: Replication — how a real directory keeps distributed copies in sync.