Again, it's a scale for maximising profit. As mentioned elsewhere, all of these formats cost relatively the same amount to print and bind, so the earlier, more expensive formats make more money for the publisher, and offer the consumer a choice, both in format and early access.

Usually, the hardback and the larger paperback are released together, the paperback being slightly cheaper. Then the B-format paperback comes some time after that. There used to be an A-format, or 'mass market' paperback that was the cheapest of all, but those have fallen out of fashion.