ordered_members vs. members and their interactions

members.rb

# How do ordered_members and members behave and interact

# Setup
col1 = Hydra::Works::Collection.new(id: 'c1')
wrk1 = Hydra::Works::Work.new(id: 'w1')
wrk2 = Hydra::Works::Work.new(id: 'w2')
wrk3 = Hydra::Works::Work.new(id: 'w3')
wrk4 = Hydra::Works::Work.new(id: 'w4')

# 1. adding to members does NOT add to ordered members
col1.members << wrk1
col1.members.size                   # => 1 -- w1
Array(col1.ordered_members).size    # => 0

# 2. adding to ordered_members does add to members
col1.ordered_members << wrk4
col1.members.size                   # => 2 -- w1, w4  (order unimportant)
Array(col1.ordered_members).size    # => 1 -- w4

# 3. adding the same object again to ordered_members does NOT add to members
col1.ordered_members << wrk4
col1.members.size                   # => 2 -- w1, w4  (order unimportant)
Array(col1.ordered_members).size    # => 2 -- w4, w4  (precise order)

# add a few more to work with for deletes
col1.ordered_members << wrk2
col1.ordered_members << wrk3
col1.members.size                   # => 4 -- w1, w2, w3, w4  (order unimportant)
Array(col1.ordered_members).size    # => 4 -- w4, w4, w2, w3  (precise order)

# 3. deleting from members does NOT effect ordered_members
col1.members.delete(wrk3)
col1.members.size                   # => 3 -- w1, w2, w4  (order unimportant)
Array(col1.ordered_members).size    # => 4 -- w4, w4, w2, w3  (precise order)

# 4. deleting from ordered_members does NOT effect members
col1.ordered_members.delete_at(2)
col1.members.size                   # => 3 -- w1, w2, w4  (order unimportant)
Array(col1.ordered_members).size    # => 2 -- w4, w4, w3  (precise order)

What is a use case for a partial_ordered list?