CardDAV is used to distribute contacts and synchronize them between different devices using a central vCard repository. If you want to access full address books offline and have access to them from different devices CardDAV is the way to go.

LDAP is like a database which you can search for contact information. LDAP can be useful only when you rely mostly on contact search than having a local copy of the same, this can be particularly useful when there is a large collection of contacts but you need only a few at a time. LDAP is also useful when you do not want to expose all contacts in the address book to the user which is specially true in an enterprise. Direct LDAP access is generally not allowed in an organization or is allowed within WAN or via VPN.