When to use the Resource usage and Task usage views?
Resource Usage is one of the assignment views, just like the view Task Usage. These views are basically the same, as they show e.g. hours/remaining availability per day per assignment.
When to use the resource usage and the task usage views?
Use these views if you need to deviate from the default 100% on assignment units. For instance if you want people to work 50% of their time to one task and 50% on another task.
Also use this view if you want to learn more about the assignment, like the work contour, assignment units or peak units.
Good to know
Note that if you change the timescale of this view to for instance months, you get a pretty good overview of the amount of hours you have already scheduled. By right-mouse-click > overallocation or remaining availability, you see how you are doing compared to your resource availability.