First, why? Philosophers want to know how good the claims are for "knowledge" in various fields. So we have philosophy of science, philosophy of mathematics, philosophy of logic, and so on. This is one reason for pursuing a philosophy of history: what is the status of specialists' knowledge of the past? What methods exist for arriving at knowledge about the past? How broad or narrow is the range of uncertainty about different kinds of historical claims?
In a similar vein, philosophers are interested in exploring and resolving some of the difficult conceptual problems and assumptions that arise in various areas of thought. There are many such puzzles in historical claims to knowledge: What are "contingency" and "necessity"? Are historical beliefs "objective" or "biased"? What is the relation between history and memory? Are there "periods" and "epochs" in history? Are ther civilizations and peoples?
Some philosophers have also pursued questions about the content ofhistory itself. Does history have direction or meaning? Does history shape a people? Is history an integrated fabric with common processes, or is it an amalgam of distinct and unrelated events? Is there such a thing as "the Russian Revolution" or "the Great Wall of China"?
So there is a reasonable subject matter for the discipline; but what is wrong with the philosophy of history we currently possess? First, writings on this subject don't really add up to a coherent and reasonably comprehensive set of ideas. Certain topics have grabbed the stage--Are there laws in history? What is a narrative? Is history teleological?--and have refused to give the spotlight to the other characters. So we might say, we need fresh thinking by talented philosophers who can re-identify a leading set of topics for discussion.
But second, and more fundamentally, philosophers have engaged "history" at too great a distance from great historians. Read any really excellent piece of historical writing today--Spence, Schama, Darnton, Bloch--and you will be struck by a raft of interesting philosophical and conceptual issues. And a new philosophy of history needs to incorporate as much of this range of working historical reasoners as possible.
Finally, why does it matter? It matters because history matters. At any point in time we are created, influenced and formed by our histories. And philosophy reasonably should shed some light on this fact.
(See The Philosophy of History in the Stanford Encyclopedia of Philosophy for more on this subject.)