why use pam_ldap in the session service?

One of our RHEL3 servers started segfaulting when using su; it turns out it was because of the line

session  optional   /lib/security/$ISA/pam_ldap.so

in /etc/pam.d/system-auth, which Red Hat’s authconfig places there by default.

So I tried to google for what session is for, and why you’d want to put pam_ldap in there, and came up with very little: this page from 2001 gave some hints but didn’t actually tell me what pam_ldap does for the session. Everywhere else on the internet just says “Oh add this line to your pam config.” No-one seems to know why.

So, for the meantime, it’s commented out, and things look like they’re working.

But I’m still curious; why does every PAM+LDAP guide say to do this?