Basically the idea is that if you have a lot of data, HDDs have much bigger capacities for the price, whereas large SSDs can be expensive. SSDs have gotten cheap, but you can get used enterprise drives on eBay with huge capacities for incredibly cheap. There’s 12TB HDDs for like $100. 12TB of SSDs would run you several hundreds.
You can slap bcache on a 512GB NVMe backed by a 8TB HDD, and you get 8TB worth of storage, 512GB of which will be cached on the NVMe and thus really fast. But from the user’s perspective, it’s just one big 8TB drive. You don’t have to think about what is where, you just use it. You don’t have to be like, I’m going to use this VM so I’ll move it to the SSD and back to the HDD when done. The first time might be super slow but subsequent use will be very fast. It also caches writes too, so you can write up to 512GB really fast in this example and it’ll slowly get flushed to the HDD in the background. But from your perspective, as soon as it’s written to the SSD, the data is effectively commited to disk. If the application calls fsync to ensure data is written to disk, it’ll complete once it’s fully written to the SSD. You get NVMe read/write speeds and the space of an HDD.
So one big disk for your Steam library and whatever you play might be slow on the first load but then as you play the game files gets promoted to the NVMe cache and perform mostly at NVMe speeds, and your loading screens are much shorter.