Using virtual destructors, you can destroy objects without 
knowing their type - the correct destructor for the object 
is invoked using the virtual function mechanism. Note that 
destructors can also be declared as pure virtual functions 
for abstract classes. 

if someone will derive from your class, and if someone will 
say "new Derived", where "Derived" is derived from your 
class, and if someone will say delete p, where the actual 
object's type is "Derived" but the pointer p's type is your 
class.

virtual destructor is recommended when you want to destroy
(delete) an object through it's parent pointer. This good
habit enforces proper cleanup of derived classes

Example:

class Employee {
  virtual ~Employee() {}
};

class Manager : public Employee {
  ~Manager() {}
}

Manager * m = new Employee();
delete m; // <--

virtual destructor is just to remove the variable without 
knowing its type.....

Dear friend , you have done  a mistake..
 Example:

class Employee {
  virtual ~Employee() {}
};

class Manager : public Employee {
  ~Manager() {}
}

 Employee * m = new Manager();//here was the mistake
delete m; // <--

Normally virtual destructor is used to destroy a base class 
object without knowing it's type...