我不明白的是我为什么要这么做

我觉得你根本不需要这么做。给定一个这样的嵌套类…

class A
{
  //B is used to help implement A
  class B
  {
    ...etc...
  }
  ...etc...
}

．.． 你总是可以移动内部/嵌套类到全局作用域，像这样…

class A
{
  ...etc...
}

//B is used to help implement A
class B
{
  ...etc...
}

然而，当B仅用于帮助实现A时，则使B成为内部/嵌套类有两个优点:

It doesn't pollute the global scope (e.g. client code which can see A doesn't know that the B class even exists) The methods of B implicitly have access to private members of A; whereas if B weren't nested inside A, B wouldn't be able to access members of A unless those members were internal or public; but then making those members internal or public would expose them to other classes too (not just B); so instead, keep those methods of A private and let B access them by declaring B as a nested class. If you know C++, this is like saying that in C# all nested classes are automatically a 'friend' of the class in which they're contained (and, that declaring a class as nested is the only way to declare friendship in C#, since C# doesn't have a friend keyword).

当我说B可以访问A的私有成员时，这是假设B有一个对A的引用;它经常这样做，因为嵌套类通常是这样声明的…

class A
{
  //used to help implement A
  class B
  {
    A m_a;
    internal B(A a) { m_a = a; }
    ...methods of B can access private members of the m_a instance...
  }
  ...etc...
}

．.． 并使用这样的代码从a的方法构建…

//create an instance of B, whose implementation can access members of self
B b = new B(this);

你可以在Mehrdad的回复中看到一个例子。

有时候，实现一个将从类内部返回的接口是有用的，但该接口的实现应该完全对外界隐藏。

举个例子，在将yield添加到c#之前，实现枚举器的一种方法是将枚举器的实现作为一个集合中的私有类。这将为访问集合的成员提供方便，但外部世界不需要/看到这是如何实现的细节。

我特别喜欢的一个模式是将嵌套类与工厂模式结合起来:

public abstract class BankAccount
{
  private BankAccount() {} // prevent third-party subclassing.
  private sealed class SavingsAccount : BankAccount { ... }
  private sealed class ChequingAccount : BankAccount { ... }
  public static BankAccount MakeSavingAccount() { ... }
  public static BankAccount MakeChequingAccount() { ... }
}

通过像这样嵌套类，我使第三方不可能创建他们自己的子类。我可以完全控制在任何bankaccount对象中运行的所有代码。我的所有子类都可以通过基类共享实现细节。

我不明白的是我为什么要这么做

我觉得你根本不需要这么做。给定一个这样的嵌套类…

class A
{
  //B is used to help implement A
  class B
  {
    ...etc...
  }
  ...etc...
}

．.． 你总是可以移动内部/嵌套类到全局作用域，像这样…

class A
{
  ...etc...
}

//B is used to help implement A
class B
{
  ...etc...
}

然而，当B仅用于帮助实现A时，则使B成为内部/嵌套类有两个优点:

It doesn't pollute the global scope (e.g. client code which can see A doesn't know that the B class even exists) The methods of B implicitly have access to private members of A; whereas if B weren't nested inside A, B wouldn't be able to access members of A unless those members were internal or public; but then making those members internal or public would expose them to other classes too (not just B); so instead, keep those methods of A private and let B access them by declaring B as a nested class. If you know C++, this is like saying that in C# all nested classes are automatically a 'friend' of the class in which they're contained (and, that declaring a class as nested is the only way to declare friendship in C#, since C# doesn't have a friend keyword).

当我说B可以访问A的私有成员时，这是假设B有一个对A的引用;它经常这样做，因为嵌套类通常是这样声明的…

class A
{
  //used to help implement A
  class B
  {
    A m_a;
    internal B(A a) { m_a = a; }
    ...methods of B can access private members of the m_a instance...
  }
  ...etc...
}

．.． 并使用这样的代码从a的方法构建…

//create an instance of B, whose implementation can access members of self
B b = new B(this);

你可以在Mehrdad的回复中看到一个例子。

其目的通常只是限制嵌套类的范围。与普通类相比，嵌套类具有私有修饰符的额外可能性(当然也是受保护的)。

Basically, if you only need to use this class from within the "parent" class (in terms of scope), then it is usually appropiate to define it as a nested class. If this class might need to be used from without the assembly/library, then it is usually more convenient to the user to define it as a separate (sibling) class, whether or not there is any conceptual relationship between the two classes. Even though it is technically possible to create a public class nested within a public parent class, this is in my opinion rarely an appropiate thing to implement.

嵌套类对于实现不应该公开的内部细节非常有用。如果你使用Reflector检查Dictionary<Tkey,TValue>或Hashtable之类的类，你会发现一些例子。