How does knowing someone is on site lead to an increase in anything production wise? All they'd have to do is wander around and look busy. Some people are willing to do this over real work despite it being a task in itself.
It's more about the fact that GCs struggle to get accurate schedules around when their subs will show up, the subs are in too high of a demand (think plumbers, electricians, framers). So they ask the subs to come out and complete a job and the sub responds with, "we'll be there sometime next week." Sometimes they show up, sometimes they don't. GCs need to know when and if they are showing up.