Having an online account allows you to sync your sources from your Desktop App to the cloud. This allows you to access your sources from the web, even if you aren't using your own computer. If you are switching to a new computer, use this option to "move" your source records from one computer to the next.
Note: attached files take up significant space in your online account. To avoid using up space, uncheck the sync full-text content to prevent files from being uploaded to your online account.