Account Information is where you'll find your invoices, account usage (space your website is using), your account's primary contacts and where you enter your ID for Google Analytics.
We need to know who to contact and keep updated as we push out changes, have scheduled down-time or outages for upgrades, or become aware of something that may affect your website. You can enter several contacts, and choose if those people are billing contacts too. And, your account contacts don't have to be Site Manager administrators/users - you can add your accounting person for billing without training her on Site Manager!
Disk Usage Report
This report shows you how much of your website space you are currently using. In the detailed list, you'll be able to see just what amount of storage each page/section is using.