Subcontractors are businesses, if they're paying people to wander around on site, they have big problems.
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.
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.