The claim is that this app can check whether your web site is "down". However, checking whether a web site is really functioning isn't something that some trivial app (judging by the $800 price-tag for the entire business) can do. For example, if my web site sells something, it's effectively dead if my payment processor goes offline. However, an app to test this from an externally visible URL would need try to buy something with a credit card before it could figure this out. All the other pages on the site could work perfectly, but the site would be effectively "down".
One approach would be to have a cron job on my site that pinged the payment processor every few minutes and e-mailed or texted me if it didn't respond, telling me exactly what was wrong. (I'd still need a way to know if the whole site went offline, but that could be done with a trivial little app on my phone.)