There's a pretty solid literature on math pedagogy, and it's clear from that research that students need some way to tell when their answers are right or wrong,

There's a pretty solid literature on math pedagogy, and it's clear from that research that students need some way to tell when their answers are right or wrong, so that they can learn from the mistakes. For homework, that could mean getting problem sets back with the wrong answers marked. But it sounds like these were just practice problem sets, so the students literally had not way to tell if they were doing them right or not.