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.
Here you can change your permalinks as you like and make it SEO friendly. You will find here 4 predefined permalink structure. You can set them. There's also a box called Custom, You can define your own permalink structure here.