Prototyping. Prolog is dynamic, and has been for 50 years. Tthe compiler is liberal, the syntax minimalist, and Doing Stuff is easy, fun and efficient. SWI-Prolog has a built-in tracer (debugger!), and even a graphical tracer. You can change code on the fly, using make/0, you can dynamically load modules, add a few lines of code without leaving the interpreter, or edit the file you're currently running on the fly with edit(1). You think you've found a problem with the foobar/2 predicate?
?- edit(foobar).
And as soon as you leave the editor, that thing is going to be re-compiled. Sure, Eclipse does the same thing for Java, but Java isn't exactly a prototyping language.
Apart from the pure prototyping stuff, Prolog is incredibly well suited for translating a piece of logic into code. So, automatic provers and that type of stuff can easily be written in Prolog.
The first Erlang interpreter was written in Prolog - and for a reason, since Prolog is very well suited for parsing, and encoding the logic you find in parse trees. In fact, Prolog comes with a built-in parser! No, not a library, it's in the syntax, namely DCGs.
Prolog is used a lot in NLP, particularly in syntax and computational semantics.
But, Prolog is underused, and underappreciated. Unfortunately, it seems to bear an academic or `unusable for any real purpose' stigma. But it can be put to very good use in many real-world applications involving facts and the computation of relations between facts. It is not very well suited for number crunching, but CS is not only about number crunching.