Why do we need fibers

For Fibers we have got classic example: generating of Fibonacci numbers

fib = Fiber.new do  
  x, y = 0, 1 
  loop do  
    Fiber.yield y 
    x,y = y,x+y 
  end 
end

Why do we need Fibers here? I can rewrite this with just the same Proc (closure, actually)

def clsr
  x, y = 0, 1
  Proc.new do
    x, y = y, x + y
    x
  end
end

So

10.times { puts fib.resume }

and

prc = clsr 
10.times { puts prc.call }

will return just the same result.

So what are the advantages of fibers. What kind of stuff I can write with Fibers I can't do with lambdas and other cool Ruby features?