Student to-do lists tend to have a specific failure mode: the essay due in three weeks sits next to today's reading, both looking equally urgent, until suddenly the essay is due tomorrow and it was "on the list" the whole time (which never actually helped). A due date only does its job if it's attached to the task somewhere you'll see it in time, not lost in a flat list.
Upcoming is where assignments and exams live, each with its own due date, so the three-week essay and tomorrow's quiz don't compete for the same visual space. Today holds the handful you're actually doing right now (today's reading, the problem set, revising for Thursday), and Someday parks the "read this eventually" pile without it cluttering either view. Just try it before next term starts — get every deadline into Upcoming once, and let Today stay short enough to actually finish.