but why not apply first and make it easier and less time-consuming for them
Because you cannot control when your GC sends your transcript. And often, high schools have specific timetables and procedures. At d's school, for example, students were required to request their transcripts at least 6 weeks before the application deadline. There aren't many students who have their applications ready to go that early, and the GC could send them off the day after they're requested.