"The concept of persistent predicates provided by this library essentially implements a light-weight, simple, and at the same time powerful form of relational database (a deductive database),..."
The fundamental idea here is achieving persistence, i.e., remembering values from one execution to another in a very simple and automatic way without having to bother working directly with files or a database. This is very useful for example for remembering configuration options (saving them e.g. in ~/.foo.d/...), producing logs, maintaining mailing lists, etc., etc. It also provides the nicest interface to databases that we know of.
Does ciao support deductive database features like handling of recursive rules using tabling etc? (as done by XSB e.g?)
Tabling improves completeness, which is useful in databases and in fact in many other applications. Ciao does not support tabling yet, although it is in the plans. It does however include several execution rules (e.g., breadth first and iterative deepening) which ensure completeness (at a cost, of course).
Hope this helps!