Usually is used when something happens enough times that you can predict the outcome with a good chance of being right. And if what usually happens doesn't, it's shocking.
> He usually comes to class on time.
From this sentence, you know he comes the class on time so many times, that you can predict that next class he will be on time again.
> He usually comes to class on time, but he's not here. I hope he's OK.
From this sentence, you see how his absence from class is shocking and caused you to worry.
Often is used when something happens frequently but not necessarily predictable. It occurs 'often but not always'.
> It rain here often.
From this sentence, you know it rains there a lot but there's no reason to predict that it will rain there tomorrow.
Usually and Often used in a conversation:
Person 1: Our teacher assigns us homework often.
Person 2: You're lucky. My teacher usually assigns us homework. Out of 10 classes we were assigned homework 9 times. Now, I just assume that I'll have homework.
Person 1: Out of 10 classes we were assigned homework 6 times. We get homework often but not always.
Both Person 1 and Person 2 are frequently assigned homework by their teachers. However, Person 2 is assigned homework so many times that it becomes predictable.