The Account tab is represented with a tree icon () because your account has a tree structure, meaning there is one root node representing your whole account, and multiple branch nodes coming out of it and representing the multiple account categories. Each category can also have multiple subcategories.
You usually need to target campaigns to specific content on your site, customize ad insertion by any part or node of the site, and most importantly - be able to break down the reporting by part of site. You might even have multiple sites where you need to control the ad serving. All of these tasks are available from the Account tab. You may want to syndicate content from a Content Partner such as Disney, to your video player, with the obligation to share the advertising revenue with this partner. There is a separate section on your Account where you can manage these types of content partnerships, which is described further down.