I would say it matters. From a UI standpoint it doesn't but from a security standpoint it certainly does.

I'm trying it out and when I go from screen to screen I can see that my URL doesn't change and there is a viewstate field which Web Forms uses. I assume OutSystems stores variables here since the URL doesn't change from screen to screen (aggregate screen, details/forms screen, etc).

Once web forms becomes completely deprecated and MS goes completely to MVC/RazorPages, viewstate won't be available anymore, which means Outsystems will start either using hidden fields to pass data from screen to screen or, more likely, they'll have to pass this data in the URL. Granted, you should always check any values that are passed from form to form, but sometimes you don't want things like ID's blatantly visible to end users either in the querystring or in hidden fields which can be easily modified using a browser's developer tools