Assets are pieces of content that are saved in a central location that can be used on multiple pages or on multiple sites. They should be used when you have identical bits of content on more than one page. If the asset content is changed, that change is applied to all pages using it. You do not have to try and find all the pages with certain content or risk missing a page and having conflicting/different information across your website.
This asset is used for entering formatted text or media items. You might create a snippet (such as a Sidebar Right: Box), linked text, content that includes images or videos, or other formatted content. Content is created in an editor similar to the TinyMCE editor that is used for content.