It's a way of ensuring you're on a supported platform with a supported browser, and, most likely, that you're only logging-in once (or in only one place at a ti

It's a way of ensuring you're on a supported platform with a supported browser, and, most likely, that you're only logging-in once (or in only one place at a time) with any given set of credentials Since it's an authenticated (from their perspective) add-on, they also might do some "clever" things with the materials being offered (animations vs flat PDFs, for example) It may also handle/verify some aspect of their support model