It's all about the presentation. It's going above and beyond the boring envelope and packaging it in a way that shows the recipient you took time to personalize it and make it pretty for them.
The card in the middle opens up with a pocket for the gift certificate on the left and the sentiment on the right.
Won't they appreciate the gift certificate regardless?
Will they even care?
Does it matter?
Yes, yes, yes, and yes.
The answer to those questions is yes, but wouldn't you rather receive something like this than a boring white envelope?
When it doubt, always craft it out!!!
(Just a personal motto...)