autosaves, opening additional files, needing to load fonts/ctb files/print settings/loading help menus - it all goes to the storage drive. As Saracen says paging also goes on, and then what happens when you want to sleep/suspend/hibernate.
As people say SSD give much better responsiveness. To stick with the car analogy you get away from the lights quicker, it has more "torque" to shove you back in your seat even when going uphill. I would liken it to clearing the road of cyclists, pedestrians and speed bumps so you get a clear run without having to slow down all the time. Now if you have all year to get where you're going you might not care. Fine spend it on 4TB HDD instead. If you need to get stuff done and find constantly waiting for ACAD to open that block in editor then get an SSD and enjoy the zippiness.
But that's the point - you don't have to be rich anymore. A decent 120GB EVO is about £70, and a 250GB £130. Money well spent
IMO, but each will have to make their own choice.