The problem is, that it's really hard to figure out. The only answer that seems to make sense is that work is measured in the same way that other kinds of value are measured: In dollars. The value of a dollar's worth of work is one dollar.
Or it could be measured according to the value added per unit of factor input, which in an efficient labor market, is equal to your salary. But we don't live in an efficient market, and nobody has a foolproof way to measure your work.
So your boss measures your work by the extent that you seem to be fully occupied. It's not a great measure, but is something that he can at least wrap his head around, and more or less keep track of.
TLDR: I don't have a good answer to this puzzle.
At my day job, my own productivity comes in bursts, and so I would also be in a predicament if my boss noticed that I was idle a lot of the time.
For some magical reason, I stopped being a disruptive student when I moved to a school where the approach was that if I finished my work I could read a book, rather than being given more work...
I usually hear that you get rewarded with more responsibility and such, but the increase in responsibility is rarely matched with a fair increase in compensation given that more responsibility usually means some combination of greater workload, more effort, longer hours, more stress.
*assuming you're being paid a decent wage and we're not talking about work being done to extraordinarily tight deadlines for no reason, in unusually poor working conditions, or something like that.
If you take a worker who operates efficiently to increase their own personal time and increase their workload without providing some offsetting "carrot." The worker has two rational responses in a liquid employment market:
1) Reduce their efficiency to median company levels and only use their efficiency "secretly." Congratulations, you've broken their spirit!
2) Find another job because they're an above average employee.
My point is just that there's no real incentive to work more efficiently (eg by automating or by smartly planning ahead) and my employer is happy as long as I stay ahead of milestones and generally work well, and its somewhat demoralising knowing that there's an infinite stream of work and no matter how hard I push myself, this doesn't get smaller, I don't get to leave earlier etc. This can be especially frustrating when other people on my team who are being compensated roughly the same as me are not pushing as hard. Why should I?
(In case someone who knows me is reading: this isn't actually the case, I'm asking hypothetically for the sake of this discussion)