Display Widgets is a WordPress Plugin that will allow you to display a specific widget to people that are logged in or people that are logged out, or may be display an additional Menu on a specific part of the site.
By using this plugin you can have different sidebars for different parts of the sites and even for people that are logged in/out of your site and will give you as a developer a lot more options for example you can show an event calendar widget to your subscribers and not to everyone else or even make your site look like that it is two different sites in different part of it, so if you have a client that has two different businesses that are connected like Hiring cars, Selling secondhand cars then you can use this plugin to have one installation of wordpress that will serve both sides of the business.
This plugin for wordpress is one of those plugins that are not for every website but if you wanted to you can use it in every website and find something for it to do(if you like playing with plugins and the look of your site).