Trying to understand the point of Gary Bernhardt's "Boundaries" talk

original.rb

describe Sweeper do
  context 'a subscription is expired' do
    let(:bob) { double(active?: true, paid_at: 2.months.ago) }
    let(:users) { [bob] }
    before { allow(User).to receive(:all).and_return(users) }
    
    it 'emails the user' do
      expect(UserMailer).to receive(:billing_problem).with(bob)
      described_class.sweep
    end
  end
end

class Sweeper
  def self.sweep
    User.all.select do |user|
      user.active? and user.paid_at < 1.month.ago
    end.each do |user|
      UserMailer.billing_problem(bob)
    end
  end
end

refactored.rb

describe Sweeper do
  context 'a subscription is expired' do
    let(:bob) { double(active?: true, paid_at: 2.months.ago) }
    
    it 'emails the user' do
      expect(ExpiredUsers.for_users(users)).to eq [bob]
    end
  end
end

class ExpiredUsers
  def self.for_users(user)
    users.select do |user|
      user.active? and user.paid_at < 1.month.ago
    end
  end
end

class Sweeper
  def self.sweep
    ExpiredUsers.for_users(User.all).each do |user|
      UserMailer.billing_problem(user)
    end
  end
end

My accounting stuff is too Reverb specific to post, but at some point in the future if I have a good example of this type of refactoring, I'll make a post on the Reverb Dev Blog