This is a convenient Ruby trick I often use for things like states:

class Auction < ActiveRecord::Base
  STATES = [
    NEW_STATE = "new",
    PUBLISHED_STATE = "published",
    SOLD_STATE = "sold",
    UNSOLD_STATE = "unsold"
  ]

  validates :state, inclusion: { in: STATES }

  def publish
    self.state = PUBLISHED_STATE
  end
end

Or perhaps:

class Auction
  class State
    ALL = [
      NEW = "new",
      # …
    ]
  end
end

Another use case might be listing DNA nucleotides:

class DNA
  NUCLEOTIDES = [
    GUANINE  = "G",
    ADENINE  = "A",
    THYMINE  = "T",
    CYTOSINE = "C"
  ]
end

If you type GUANINE = "G" into irb, you'll see => "G". That means the assignment expression returns "G" as its value.

We simply take that value and stick it in an array assigned to another constant.

In Ruby, almost everything is an expression and every expression returns a value, including things like assignments, conditionals and class definitions.

That same principle is why this works without explicit returns:

def my_method
  if condition
    "One return value"
  else
    "Another return value"
  end
end

The value of a string is itself, the value of a conditional is the value of its realized branch, and calling a method will implicitly return the last value of its body.