I have to wonder if he was thinking about, for example, a reusable implementation of a hash table. And if we was, why in the world wouldn't he want that. Running with the hash table example: I use them many, many times a day, and if I had to reimplement them every time, I'd be sunk. Just a few moments ago, I wrote a script to check for duplicate entries that looked something like this:

  seen = {}
  elements.each do |element|
    if seen[element]
      raise "Duplicate element: #{element.inspect}"
    end
    seen[element] = true
  end

It's quick and dirty and isn't a shining example of architecture. But it found my duplicates and let me move on with my day. But what if I didn't have a reusable hash implementation to lean on? Would I even have attempted to write that script? Or would I have done my duplicate checking manually, wasting about an hour?