I never saw the practicality of them.
I wear a belt to keep my pants up. If I had to undo my belt to get my knife, my pants would be falling down.
I also don't see the sense of them for self-defense. Not only for the reason I just mentioned (pants falling down) but I don't imagine it would be all that easy to undo ones belt buckle when being assaulted. And I doubt a bad guy is just going to stand around and wait while their intended target undoes their belt.
And of course, the mere possession of such items (belt buckle knives) is illegal in my state of California.
To me, they seem more novelty than practical.