In gifting a book you are gifting a *specific* title you choose.
In gifting a gift card you are gifting *money* and, in effect telling to recipient; "Buy a book. Any book."
In the former case you're basically recommending a specific book you think the recipient will enjoy. In the latter case you are farming out the selection process to the recipient. Less effort, less thought goes into it.
Any way you look at it it's a big difference.
Oddly, no mention that Kobo also allows gifting.
The even *announced* it a day before Amazon went live.
(Of course, the announcement was that the feature was coming, not that it was live, but still...)