One of the core features of views is the ability to filter rows according to particular criteria. When a Row is filtered, it is not deleted—it's just hidden from the particular View you're using and remains within the table (Views are virtual tables!).
Try adding filters when:
You need to focus on rows that meet certain conditions (e.g. Classes between May and June, and taught by Albert Einstein).
You need to display certain information, to certain people (e.g Each sales person can only view their own commission).