A Sidebar is a separate content area in a WordPress theme which may contain Widgets. Sidebars and widgets can only be managed by Site Editors.
This site comes with a Global Site Sidebar which is included on all single and archive displays. By default, the Global Sidebar contains no widgets, so it is not displayed.
So why have it? Because since it exists on all posts, pages, and archives, we can inject separate custom sidebars to override it wherever we want with very specific control.
For example, I have built a custom sidebar called “Blog Sidebar” which overrides the Global Sidebar on all posts in the “Blog” post category and “blog” category archive pages. This custom Blog Sidebar does contain widgets, so it is displayed.