Earlier quoted context omitted.
The problem is when the grades given on assignments influence what goes on your final exam papers. At a university, only the final exam should count. This way you can give honest feedback, and the student doesn't feel obligated to point out every little mistake to try to make you change your decision. It also allows them to misunderstand things at first, but then 'get' it later. Of course universities don't like this…
My experience was that the top students (now PhD students at MIT, Stanford, etc) were very diligent about homework. In the honors classes, you often needed to go far above and beyond the textbook for some questions. The average students (still smart kids, many working at Google, Amazon, etc) were much more lax about it, and wouldn't work that hard to solve the hardest problems in a set.
Living in the dorms helped enormously with this, because I could usually find someone to help me with particular problems, and I sorely needed that help.
Anything less meant I was in deep doodoo on the exam :-) because that's the wrong time to try to figure out how to apply what was learned in lecture.
This was good enough to reliably get a B. To get an A, I had to work much harder and go beyond the homework.