What's the Comparer class for?

Question

What purpose does the Comparer class serve if the type that you specify already implements IComparable?

If I specify Comparer.Defaul

Answer 1

public class Person
{
    public string LastName;
    public string FirstName;

}

public class Class2
{
    public void test()
    {
        List classList = new List();
        //add some data to the list
        PersonComparer comp = new PersonComparer();
        classList.Sort(comp);
    }
}

public class PersonComparer : Comparer
{

    public override int Compare(Person x, Person y)
    {
        int val = x.LastName.CompareTo(y.LastName);
        if (val == 0)
        {
            val = x.FirstName.CompareTo(y.FirstName);
        }
        return val;
    }
}