It depends on your view point. A lot of art is critized for the artist's abilities and techniques to create the art and much less for the subject matter. The David could have been any random dude, but it is the ability of accurately recreating that randome dude in stone that makes it impressive. Some of the painted pieces that are extremely large and take up an entire museum wall are impressive just for the shear accomplisment. Yes, it helps that it looks so real as well.

>See also: displaying unfinished sketches. Nice for a collector but let's not pretend there's any artistic value in it.

If you imagine those sketches being underneath the painting and realizing that those sketches were how the artist worked on creating the actual piece is interesting in the same way watching behind-the-scenes videos on how a movie was made are interesting--to those that are interested in the first place. Similar to Show HN articles on some use of fad language to redo something else already in existence. If you have no interest in that language or the thing being made, they aren't interesting.