I'm surprised nobody is pointing out similarity to Intel's very successful Tick-Tock release cycle - release a new architecture on tick and refine the process on tock
The benefits for Apple are manifold, but the two most obvious are:
1. Keep expectations sane - they can't release a radical new industrial design of superbly high quality every year.
2. Give customers on a 2 year contract predictable value rather than leaving them always waiting for the next awesome device
Windows 95 was one of the most successful product launches of all time. And Windows 2000 was a very solid product though not generally a good choice for casual users.
The benefits for Apple are manifold, but the two most obvious are:
1. Keep expectations sane - they can't release a radical new industrial design of superbly high quality every year.
2. Give customers on a 2 year contract predictable value rather than leaving them always waiting for the next awesome device