为什么SelectMany需要M<S> -> (S -> M<C>) -> (S -> M<C> -> R) -> E<R>之一？

Question

在链接https://msdn.microsoft.com/en-us/library/bb534631(v=vs.110).aspx中，第三个签名是

// M<S> -> (S -> M<C>) -> (S -> M<C> -> R) -> E<R>
public static IEnumerable<TResult> SelectMany<TSource, TCollection, TResult>(
    this IEnumerable<TSource> source,
    Func<TSource, IEnumerable<TCollection>> collectionSelector,
    Func<TSource, TCollection, TResult> resultSelector
)

第一个具有典型Monad绑定函数M<A> -> (A -> M<B>) -> M<B>签名的函数还不够吗？将resultSelector中的代码合并到collectionSelector中不是很容易吗

MSDN文档给出了一个示例来说明该方法的用法。

class Program
{
    static void Main(string[] args)
    {
        SelectManyEx3();
    }
    public static void SelectManyEx3()
    {
        PetOwner[] petOwners =
            { new PetOwner { Name="Higa",
                    Pets = new List<string>{ "Scruffy", "Sam" } },
                new PetOwner { Name="Ashkenazi",
                    Pets = new List<string>{ "Walker", "Sugar" } },
                new PetOwner { Name="Price",
                    Pets = new List<string>{ "Scratches", "Diesel" } },
                new PetOwner { Name="Hines",
                    Pets = new List<string>{ "Dusty" } } };

        // Project the pet owner's name and the pet's name.
        var query =
            petOwners
            .SelectMany(petOwner => petOwner.Pets, (petOwner, petName) => new { petOwner, petName })
            .Where(ownerAndPet => ownerAndPet.petName.StartsWith("S"))
            .Select(ownerAndPet =>
                    new
                    {
                        Owner = ownerAndPet.petOwner.Name,
                        Pet = ownerAndPet.petName
                    }
            );

        // Print the results.
        foreach (var obj in query1)
        {
            Console.WriteLine(obj);
        }
    }
}

class PetOwner
{
    public string Name { get; set; }
    public List<string> Pets { get; set; }
}

但是，可以使用第一个签名的SelectMany重写var query = ...，如下所示？

var query = petOwners.SelectMany(o => o.Pets.Select(p => new { petOwner = o, petName =p}))

带第三个签名的SelectMany什么时候会真正有用呢？