Being able to organize different assignments by subject area or Unit helps students know exactly where to go when using google classroom. For example, I would post online math games for students and they always knew it would be in the "Math" topic. Teaching students how to recognize they have submitted an assignment or not, is also useful when they double check before logging off. When I taught remotely for the year, I used multiple topics and was posting multiple assignments weekly so it was crucial to be able to organize my google classroom and easily navigate between subjects and materials for both myself and my students.
