为什么析构函数虚函数？源代码

// Destruct.cpp : Defines the entry point for the console application.
//
#include "stdafx.h"
#include <iostream>
using namespace std;
class A
{
public:
	A()
	{
		cout << "A::A()" << endl;
	}
	virtual ~A()
	{
		cout << "A::~A()" << endl;
	}
};
class B:public A
{
public:
	B()
	{
		cout << "B::B()" << endl;
	}
	virtual ~B()
	{
		cout << "B::~B()" << endl;
	}
};
int _tmain(int argc, _TCHAR* argv[])
{
	A* pA = new B;
	delete pA;
	return 0;
}