From a small employer perspective, good employees do more than assigned. It's impossible to name and identify everything you want done within a certain amount of time. There is always more work than hours in the day to do it. Maybe corporate America is different (if so, I guess they're paying for redundancy, which isn't necessarily a bad thing).
From my experience, though, those who say "but you said I only had to do this and I finished now I want to go home" are the ones you get rid of. You want people who understand their "job" is to make the firm successful and I'm paying you to do that 40 hrs a week. If you do a better job at it than others, then I'll pay you more. The best employees I've ever had understood this and did stuff all the time I had not assigned them. Other people I know who own small businesses have expressed the same sentiment.
I'd also guess that if you're a completely transactional type employee focused on only doing a very specific assigned task, you risk being replaced that much more easily by AI or tech or whatever that is cheaper. If you're the type who has a more general outlook like I described above, a good employer will do anything they can to keep you.