Why just use a floating shelf instead of a wall niche?
The main difference is, of course, the beauty. Using a wall niche is a way to truly showcase not only what is inside the niche, but also the wall that it is located on. It is an interesting concept of the niche turning your wall into a work of art whiles the niche and the art work together to enhance each one's appearance. If you would simply display your art and trinkets on a floating shelf, then it would just get lost and seem as if you were trying to either fill a shelf or just haphazardly throwing something on a shelf to get it out of the way.
A wall niche is something that will definitely make the item in it look as if it has a specific purpose and it will let the true potential of that item shine through. The best part of it is that it is almost fool proof - with millions of possible combinations, you can't go wrong!