You can create a sidebar and then select it as the default sidebar to appear on all posts or all pages.

This can be useful for promoting a particular campaign or call to action on specific, related content. If you want a different sidebar for a specific page or post:

Sidebars are a great way to give users options for staying on the site and exploring more of your content. They can also be used for conversions with call to actions for donation, joining as a member or taking action.

Sometimes you want to create a layout on a post which makes use of the full width of the page. To do this you will need to remove the sidebar.