Here’s how it works:
You tell them what DVDs you want to watch, and they post them to you. When you’re done, you post them back, and they send you another from your list to replace it. Easy Peasy Lemon Squeezy.
Just like Netflix, they have a convenient queuing system, where you rank your films by the order in which you’d like to watch them. When you’ve seen a film it gets removed from the queue. If all their copies are out, they send you the next one down your list instead.
Oh, hold on, no they don’t. They’ve just changed it to a much less useful system instead.
Why would they do a thing like that?
One possible explanation is that Netflix has patented the idea of a queue.
Which, in the US at least, means that no-one other than Netflix can offer this feature (without paying money to them) for another twenty years.
Software patents stifle innovation.