A hybrid.. aka a SSHD is using a SSD cache mechanism to 'cache' data transfers for slower 5400RPM drives to make them as 'fast' as a more expensive, more heat, more power consuming, heavier 7200RPM. Both are still MECHANICAL drives, not RAM based drives like a SSD. A SSD uses ONLY RAM-like chips that can LONG TERM store data, even when powered off (your RAM Memory your PC uses doesn't do this, it clears itself every-time it shuts down). A SSD has NO MOVING PARTS, so there is 'instant' access to the stored code (Windows, Drivers, etc.) and is very light weight, takes up less room, uses barely any power to do the SAME job as a HDD, and creates relatively no heat. Thus making it perfect for portable systems like your Smart Phones, Tablets, and then Laptops.
Desktops can use them as a 'side effect', which thus turned open a whole new market with the virual videos showing 7second from COLD OFF PC to Full Desktop and icons loaded and ready for use Windows PCs.