Leather stitching, etc. are not needed. But a small shadow or edge here and there is useful for how human brains perceive buttons and other interface elements from things that are not.
Color too is not just decorative or fashionable but also important in how it associates with interface elements, organization, and the over all UX.
In a well designed interface, we might normally detect shadow, form, and color but all these things combine to actually, in theory and often in practice, make it easier to use a device or screen interface.
If you notice the decorative excesses (no matter how minor), heavy shadows, over done dimensionality, etc. the interface failed. If you have trouble using it because it is too flat or colorless or lacking in color range, it has failed.
But, if it just works, and you don’t notice things one way or the other, it has succeeded.
Within any interface, there are exceptions. The proverbial “abandon ship” button should probably be big, colorful, and a touch too big. All buttons and links should also be big enough for that mythical above average human finger to actually touch. In iOS, my average human male fingers often cannot hit things as they are too small. But often it is placement that matters. The CNN app back button is a very tiny circle with an arrow in it, but it is placed in a spot where it works every time. In any app that uses a keyboard, the small default keyboard delete button is too close to the edge of the screen and too close to another button. I have deleted or not deleted too many times to count.
In early web design projects 10 or more years ago, I would often have to remind the data base programmers (I was the art director) that having some white space around text on pages and forms was essential. It seems that same problem afflicts app development too and since touch and buttons are at play, it is even more critical but somewhat unrecognized as important for the UX.