You can use ScanCloud without signing up using the anonymous keys but to take full advantage of the service you will need your own keys.
An API key gives you both privacy and choice.
Privacy - When you submit a file, an email or an URL to ScanCloud for analysis, if you use the default anonymous key, anyone who visits the
Activity page of the ScanCloud website can see the file name, url or the subject of the email you had submitted for analysis. But when you register an API key with ScanCloud and use it in your applications, the submissions made by those applications and the results of such submissions are visible only to you.
Choice - When you do not create an API key of your own, the analysis is limited the default vendor choices selected by ScanCloud. Though the results themselves are highly accurate, it might not be your preferred vendor for a variety of reasons. When you register an API key, ScanCloud lets you choose which vendor your data to be scanned with.
Management - From within your account in the ScanCloud Portal you can view activity, see reports and change configurations for each of the keys that you have created.