not understanding return statement

Hi all, I need help understanding part of my code. I'm learning about data
structures, at the moment binary search tree.
This code does what I want, but I don't understand return statement in
insert
method. Why @root = root works as it works. If i put last line just root,
global variable root is nil instead of tree filled with nodes.
Here is my code:
class BinarySearchTree

class Node
attr_accessor :value, :left_child, :right_child
def initialize(value)
@value = value
@left_child = nil
@right_child = nil
end
end

attr_accessor :root
def initialize()
@root = nil
end

def insert(root = @root, value)
if root == nil
root = Node.new(value)
elsif value <= root.value
root.left_child = insert(root.left_child, value)
else
root.right_child = insert(root.right_child, value)
end
@root = root #works with this return statement
#root <- doesn't work with this statement, why!!!
end
end

x = BinarySearchTree.new()
x.insert(15)
x.insert(10)
p x.root


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