My guess is that they build in a small SSD (likely the equivalent of an SD card) to be used as a cache.
The thing is that Windows 7, and later Linux kernels can use the SD card as a cache for commonly used small files.
Where this helps is in reducing latency on reads. So for example, there may be hundreds to thousands of small DLL files needed to start and run the OS, but they are scattered all over the disk. Regardless of defragmentation, they still need to be found each time.
An SD card might not be all that super fast on reads, compared to a 15K spinning drive, however the seek time is still WAY faster.
With Win 7, all you need to do, to do this on your own is pop in an SD card, a USB stick, whatever, and one of the options you will get asked, is whether you wish to use ReadyBoost, and if so, how much space on the device do you wish to use. It makes a monolithic ReadyBoost file as big as you specify.
However I have heard that the benefit does not help much beyond 2-4 GB. So for the price of a 4 GB SD card (less than $5 around here) you can test this performance booster.
If you already have an SSD drive for the boot driver, then Windows will not offer ReadyBoost as it offers no benefit in that use case.