The banner is typically used for branding i.e. the company’s logo and it’s selected in the Design options. But you can override it for specific pages if you want.
If the banner is using the default setting in YWD Designer, it will cascade automatically from any page to its sub-pages, so it's not necessary to select a banner for every page.
Any image you have uploaded to the Assets library will be available from this banner tab. In this tab of the Page properties you select the asset that will be used for the banner. Although you can specify a default banner image when designing your website in YWD Designer, you may use the page properties to choose a different banner for a page. For instance, you may have a separate banner for each section of the website, or you may want to have a "tall" banner in the Home page and a thinner one throughout the other pages of the website.