Hybrid portion of an SSHD can range between 8GB to 32GB+ NAND flash memory, the idea for these drives is to store most frequently used programs in this buffer to quicken things. The cache size isn't very big and wouldn't expect much in gaming sense since modern games are quite large, not sure how a game 30odd gig would go with an SSHD.
People have HDDs for storage because SSDs were pretty small and quite expensive, it's been a longish trend and advised for this setup configuration, ie 256GB< capacities. SSD's are now 50c a GB making the price for 500GB/1TB not that bad.
It's not uncommon for people to have more than one HDDs for various reasons. For a typical gamer, you'd have a HDD to store your game backups/movies/music/downloads...