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Why use virtus with rails form objects?

When I use the form object pattern in rails, I include ActiveModel and I tend to delegate attributes to an actual model.

Why would I want to use the virtus gem? Seems I'd be redefining attributes that are already defined elsewhere (i.e. a model)

How to sort virtus collection of date object?

require 'virtus'

class User
  include Virtus.model

  attribute :id, Integer
  attribute :experience, Array[Experience]
end

class Experience
  include Virtus.model

  attribute :begin_date, String
  attribute :end_date, String
  attribute :title, String

  def duration
    [begin_date, end_date]
  end
end

I have two dates:

[Experience[begin_date: '2010-10-20', end_date'2012-03-24'], Experience[begin_date: '2010-04-01', end_date: '2012-01-20']]

I need this to distinguish first and last year of user experience history.

  def experience_years
    first_job_date = experience.map(&:begin_date).compact.sort.first
    last_job_date = experience.map(&:end_date).compact.sort.last

    last_job_date.year - first_job_date.year + 1 if first_job_date && last_job_date
  end

I thought about using virtus collections like here:

class BookCollection < Array
  def <<(book)
   if book.kind_of?(Hash)
    super(Book.new(book))
   else
     super
   end
  end
end

class Library
  include Virtus.model

  attribute :books, BookCollection[Book]
end

But I am not sure how to do it.