They are, but I think the question was more "does the increased speed of an SSD make a practical difference in user experience for immich specifically"
I suspect that the biggest difference would be running the Postgres DB on an SSD where the fast random access is going to make queries significantly faster (unless you have enough ram that Postgres can keep the entire DB in memory where it makes less of a difference).
Putting the actual image storage on SSD might improve latency slightly, but your hard drive is probably already faster than your internet connection so unless you've got lots of concurrent users or other things accessing the hard drive a bunch it'll probably be fast enough.
These are all Reckons without data to back it up, so maybe do some testing