Here is my standard answer:
Intel's SRT caching technology was designed for buyers who could not justify or afford the cost of a larger capacity solid-state drive. According to Intel, the original idea was that for about $100.00 a user could purchase a small capacity ssd of about 10 to 20GB and use it as a cache to improve hard disk drive performance. The Operating system and programs were actually stored on a hard disk drive. The actual improvement could not compare to a stand alone ssd. Intel also looked at different capacities all the way up to 512GB and concluded 64GB was the point of diminishing return. It made more sense to use a 64GB ssd as a boot drive that also contained software programs. Intel was hoping that if business clients saw an increase in performance, then they would be induced to purchase larger capacity ssd's that promised an even greater boost in performance.
A lot has changed sinced then, especially prices. For $100.00 you can definitely purchase an ssd that is much larger than 20GB. Currently you can buy a 128GB SATA 3 6Gb/s solid state drive for less and have money left over. Might as well take full advantage of ssd performance.