How could I sensibly overload placement operator new?

C++ allows overloading operator new - both global and per-class - usual operator new, operator new[] used with new[] statement and placement operator new separately.

The former two of those three are usually overloaded for using customized allocators and adding tracing. But placement operator new seems pretty straightforward - it actually does nothing inside. For example, in Visual C++ the default implementation just returns the address passed into the call:

//from new.h
inline void* operator new( size_t, void* where )
{
   return where;
}

What else could it do? Why and how could I sensibly overload placement operator new?