So this is a fair question but the answer will be long. There's three basic reasons:

1. Feedback

2. Monitoring

3. Socialization

I teach at a big public university. My courses are big -- as enrollments in the major have fallen (partly macro trends, partly micro crises), they've gone from ~ 225 to ~120, but that's still an enormous number of students. When I taught at Dickinson and Georgetown, I could rely on small-group and face-to-face dynamics to provide me with information about what was happening in my classes--courses of 12 to 30 work differently. This applied even at the level of in-class lecturing or monitoring in-class discussions--I could hear and track two or three groups' conversations at a time. In my upper-div courses at UMass, I use almost the same techniques. In a lecture hall of 100+ students, that's not possible.

As an instructor, that means that in these settings I lack any idea of what’s happening in my class. To gauge that, I can wait for people to bomb the midterm, or I can keep track of what’s going on with smaller, lower-stakes assignments.

Separately, with a student population that’s more diverse (in the pre-DEI sense, I mean across all the categories) than my GU or Dickinson ones, I need ways to monitor who’s in trouble. “Trouble” here can mean the real kind, the alert-the-dean-of-students kind. Especially during the Covid period, that also meant I could focus interventions much more on students who were checked out or checking out—something that makes a huge difference for populations of students who might succeed or might not, depending.

The last part concerns my courses’ role in the curriculum. These are big courses meant to introduce folks to the discipline and to college itself. I cannot assume anything about study habits, background knowledge, understanding of the world, etc. Having weekly assignments—and these are small things, sometimes as small as a paragraph—is a means not only of keeping track of students and learning where trouble spots are. It’s also a way of transitioning students into greater independence. In many (most?) of my students’ high schools, they don’t/rarely do homework, and college is a major shock to the system. This isn’t the world of “excellent sheep”—this is a world of introducing (some) students to basic concepts. (I once had a student tell me that they had never read a nonfiction book until my class.)

This last part has become more important over time. Without graded work, there’s a category of students who will not do the readings (or come to class!). And although it’s tempting to let them sink or swim, if you sink out of UMass (or any public), there’s a nontrivial chance that you’ll sink out of higher ed. That’s a very different calculation from, say, Dickinson, where I had a support team—I yearn for it—of nine dean/assistant-dean level folks who could help me work with students in trouble and coach them through tough times. We have a (very good) student success team, but they have fewer staff for a much larger population.

Having the graded work (and it’s graded, again, on a pass/fail or check-scale basis) provides the incentive necessary to get (some category of) students to engage. I emphasize that some students would do everything without the incentive, and a few won’t no matter what. But the category of students in the middle has grown, and even if, in my upper-div courses, I can move back toward something that looks like my college experience, in my lower-div courses it would be difficult without harming (not physically but nontrivially) some proportion of students who require greater structure.