The function is measured if the work of art 1. is not visually appealing
2. is a very expensive piece of art
3. should serve a practical purpose
4. causes critics to disagree about the value of the art

i got this answer from a really friend, and it was right.

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artists need to create a realistic interpretation so that people in general can relate to the artwork. for a common people, an artist's impression gets increased only if he can create something which would look realistic. an abstract is difficult for a common people to understand whereas if a person can draw a girl with simple pencil sketch and if that looks like a real figure then the latter will be praised more than the former.