To be perfectly clear
This is a solid state CACHE, not a solid state drive. As others have mentioned, this is meant to be used in conjunction with a mechanical hard disk, not an SSD. To this end, it was built with SLC, not MLC NAND. SLC NAND last 100,000 write/erase cycles, vs. 3000 for MLC. It is expensive because it is durable. The size: 20 GB is plenty for cache. SATA II speeds are fine for cache.
This is not the same as having a RAM cache, which disappears when you power off the machine. This SS cache retains information on power down. This means it can retain the boot sector and other info for lightening fast boot ups, rather than having to access the slow hard drive with which it works conjunction.
I know you still don't get it, but I thought I'd try anyway ;-)
ZipZap