What is it in SC that needs to pull data quickly from the SSD?
This is not specific to SC, but I'll explain it in terms of SC. There is a file that SC uses, called Data.p4k. This file is compressed to 48GB. The file contains the software and models for game assets, and decompresses to over 100GB. Yes, that's over 100,000,000,000 bytes. This file is scanned frequently by SC for models used in the scene, the uncompressed version is used. No regular computer has that kind of RAM, so the disk paging file is used to help. MacOS has this capability too. It's not exempt from the laws of physics.
I'll replace the Mac hopefully before it dies, but I need to add a dedicated gaming machine, so looks like I need to understand these newfangled SSD's better.
They're essentially a huge bank of non-volatile RAM being used as a hard disk. Non-volatile means when the power's turned off, the content of the memory is preserved. It turns out one of the form factors they use looks like a hard disk. This is so that older hardware design can benefit from the technology. There are other ways of connecting them, including on some motherboards, PCI, PCI Express, and Samsung's M.2. The benefit of using these alternatives is that the length of the connection between the motherboard, and the storage device is minimized. That has a dramatic effect on the speed of the storage device, and these storage devices have data transfer rates that approach that of RAM if they are mounted on the motherboard.
The idea is to have the operating system, SC (or whatever software) and the virtual memory stored on the fastest data storage devices possible. It does not really matter if they are on the same or different devices, but PC internal architecture tends to be the limiting factor here - so we often put them on the same device. Preferably without a 2 or 3 foot cable between the storage device and the motherboard, but you generally don't notice too much. Even SC doesn't seem to be bothered by that.
My gaming rig is approaching 7 and a half years old. There is nothing wrong with it but i've slowly started looking into upgrades. New motherboards have the SSD slots built in directly to the board so they have access to the PCI bus. That should be a kick in the pants for disk performance.
If you want, you can replace your computer's hard disk with SSD and get about (up to) 75% of the gain you would get from PCI or M.2.