The obvious use case is setting a timer while cooking, when the phone is at the other end of the room and your hands may be dirty, but I'm using it even in non-cooking settings.
In fact, I just tried it and timed both approaches for setting a reminder. Results:
* Assistant: "Hey Google, set a reminder 10 days from now to file my taxes.": 4.5s to say it (I'm free to focus on other things after this point), 7.5s until reminder is created.
* Manual (pick up & unlock phone, open app drawer, open Calendar app, find the day 10 days from now, click on that day, click create a reminder, type in "file my taxes", save): 17s