I don't understand it either. I negotiated my salary for 37.5 hours a week of work. I'm happy to work a bit more when the work is interesting, but I'm going to show up late or leave early the next few days to make up for it. Alternatively, you can pay me more.

That's the deal. My work/bosses are happy with it. My co-workers seem to begrudge that they're freely giving up extra time and I'm not. I keep suggesting they stop doing that...