Well, it gets the reading remotely from the blood glucose meter and calculates the dose. It then displays the amount of insulin for the bolus delivery. You look at it and generally, if you've been using a pump or doing injections you know about what range you usually end up taking. If it's off a lot it should be obvious as long as you're actually alert. When it comes to things like that being observant is important.
Using the bolus wizard is one path through the menus but is not the only one. If you have remotely connected to the pump you can tell it to deliver without requiring the user to press any buttons. Medtronic have turned off some of the remote ability with the firmware in their later pumps, unfortunately that has also denied access to projects such as OpenAPS. I would like to see some ability to pair known devices together rather than cutting off all access completely.