There are scripts to do this type thing on Unraid, possibly adaptable to other platforms/scenarios. Search for “unraid userscript move watched to array”.

There are even ones that will move an entire season or show over to your “fast” drive whenever you start watching a show.

I don’t have anything so complex in use currently; i just have it set to keep new stuff on the fast drives for 60 days, then shuffle it off to hdds.

As far as why ssd over hdd? Prob more of a power issue than a speed issue. Depending on how your storage is setup, it can save a bit if juice not having to spin up an array of hdds.