dynamic foo = 123;
Console.WriteLine(foo + 234);
// 357    Console.WriteLine(foo.ToUpper())
// RuntimeBinderException, since int doesn't have a ToUpper method

foo = "123";
Console.WriteLine(foo + 234);
// 123234
Console.WriteLine(foo.ToUpper()):
// NOW A STRING

using System;

public static void Main()
{
    var value = GetValue();
    Console.WriteLine(value);
    // dynamics are useful!
}

private static dynamic GetValue()
{
    return "dynamics are useful!";
}

using System;
using System.Dynamic;

dynamic info = new ExpandoObject();
info.Id = 123;
info.Another = 456;

Console.WriteLine(info.Another);
// 456

Console.WriteLine(info.DoesntExist);
// Throws RuntimeBinderException

The following output equivalent results:

class IfElseExample
{
    public string DebugToString(object a)
    {
        if (a is StringBuilder)
        {
            return DebugToStringInternal(a as StringBuilder);
        }
        else if (a is List<string>)
        {
            return DebugToStringInternal(a as List<string>);
        }
        else
        {
            return a.ToString();
        }
    }

    private string DebugToStringInternal(object a)
    {
        // Fall Back
        return a.ToString();
    }

    private string DebugToStringInternal(StringBuilder sb)
    {
        return $"StringBuilder - Capacity: {sb.Capacity}, MaxCapacity: {sb.MaxCapacity}, Value: {sb.ToString()}";
    }

    private string DebugToStringInternal(List<string> list)
    {
        return $"List<string> - Count: {list.Count}, Value: {Environment.NewLine + "\t" + string.Join(Environment.NewLine + "\t", list.ToArray())}";
    }
}

class DynamicExample
{
    public string DebugToString(object a)
    {
        return DebugToStringInternal((dynamic)a);
    }

    private string DebugToStringInternal(object a)
    {
        // Fall Back
        return a.ToString();
    }

    private string DebugToStringInternal(StringBuilder sb)
    {
        return $"StringBuilder - Capacity: {sb.Capacity}, MaxCapacity: {sb.MaxCapacity}, Value: {sb.ToString()}";
    }

    private string DebugToStringInternal(List<string> list)
    {
        return $"List<string> - Count: {list.Count}, Value: {Environment.NewLine + "\t" + string.Join(Environment.NewLine + "\t", list.ToArray())}";
    }
}

The advantage to the dynamic, is adding a new Type to handle just requires adding an overload of DebugToStringInternal of the new type. Also eliminates the need to manually cast it to the type as well.