Actually using Ajax directly into your plugin or theme is a little immature way. Cause you'll have to keep track of your files to make it work, which sometime become difficult. That's where WP_Ajax make things comfortable.
Sometimes it's needed to add extra column on user table to show extra property file a custom meta field or something like this.
Let's say you want to list a column called "Customer Since" on user table which will display the registration date of your user.