Why do have to categorize art, anyway?

I guess by virtue of even asking this question, that makes me a formalist.  I love art for art’s sake.  I enjoy great cinematography, videogame character models, cool-looking buildings, anything that is aesthetically pleasing to me.  Within this realm, I am certainly not against art having deeper symbolism, whether the author/painter meant it or not.  But I do not look at a painting or sculpture and wonder what the originator was thinking while making it.  The only artforms I personally do that with are literature and film.  Even to those ends, I try not to dig too deeply, because at their core, what we are looking at in all these art forms is art.

As far as comparing post-modernism to formalism, I can attempt to do that here.  But I don’t know that it will change the way I view the art world.  There are similarities between the two.  To some people, post-modernism came from formalism; in other words, post-modernism tries to give meaning to formalist work.  I thought this concept was kind of ridiculous, especially when talking about the works of Agnes Martin.  To the modernist, she just creates aesthetically pleasing, orderly works of art using lines and open spaces.  To the post-modernist, the book says that some claim she is talking about different levels of reality.  I don’t agree.  I just think she was trying to make something cool-looking.  Let’s take some of Joel Shapiro’s work.  The first image we see of one of his works is not even titled.  Clearly it was made for art’s sake and not to convey something deeper.

The main difference between these two concepts is that formalism is a technique for creating art (as well as a way to observe it), whereas post-modernism is just a way to observe art in my opinion.  I mean, sure you can set out to make a work of art with meaning; but that would be Expressionism.  You could do Realism with social undertones; but it is still just that.  People will look at your art however they want to look at it.  They will either look at it like me (art for art’s sake), they will try to find meaning in everything, or they will observe your art in a completely different way.  None of these ways is right or wrong and they are all equally valid.  This brings me to my original question.  If there is no right or wrong way to make or observe art, why are we naming classifications of it in the first place?