I get this question all the time in CCSA classes. The TL;DR version of the answer is that as long as you have a firewall running at least R75 and a (free) CPSB-IA license, there is no need to enable UserDirectory or obtain a license for it UNLESS:
1) You want the ability to manage LDAP users in a read/write situation; in the real world this generally happens over the still-quivering dead body of your LDAP server administrator (i.e. practically never!). This will allow Remote Access VPN users to potentially change an expired password via the Check Point Remote Access VPN software as Marco Valenti observed, and also allow changes to LDAP user passwords/groups directly via the Check Point SmartDashboard (which also requires an extension of the LDAP user schema). This feature's checkbox is called "User Management" on the LDAP Account Unit object. (see screenshot below)
2) You want the ability to retrieve CRLs via LDAP instead of the more-typical HTTP or OCSP (not common). This feature's checkbox is labelled "CRL retrieval" on the LDAP Account Unit object.
3) You need to do an integration to an LDAP server that is not based on Microsoft Active Directory (i.e. Novell eDirectory, Netscape, Lotus Domino, etc). I've never done an LDAP integration to a server that was not AD in over twenty years of Check Point experience, so that should give you an idea of how common it is.
The "Use UserDirectory for Security Gateways (license required)" checkbox on the Global Properties screen for UserDirectory/SmartDirectory does NOT need to be set for IA to operate, which runs counter to some of Check Point's documentation and the CCSA R80.10 courseware. When IA's AD Query feature is first set up the wizard automatically creates the needed Account Unit object through the underpinnings of UserDirectory which is where a lot of the confusion comes in.
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