Not for gaming so much after ssds got big and cheap, but on NAS Operating Systems like Unraid and TrueNAS, (and likely others) you have a pool of hard drives that do big data storage. Like you pointed out IO is slow. So to speed things up you have an ssd setup as cache. Typically you have the programs you have running are running off that ssd cache. It does IO to the ssd and you setup a slow scheduled replication to the hdd pool.