There's a technique called page swapping which allows for large or small amounts of memory to be quickly changed to a completely different set of memory. The speed of current hard drives that most games use is nowhere near the speed that memory can function, so page swapping parts of memory in this way isnt practical. But, being able to do it at a much faster speed that ssd allows is a game changer. The only downside is that it does require more memory management, but when you start getting used to it, it works wonders. As of now, since a swap needs an equivalent amount of memory, its pointless, since you can just fill new parts of memory with the new data, and retain the old.

Exactly. It also allows easier refreshment of assets during gameplay, which is especially welcome with open world games. Anyone who's played heavily-modded Fallout 4 or Skyrim on an HDD and an SSD can attest that is a huge improvement.