Art the imitation

When talking about Art in the form generally described in the 17th century we are referring to imitation art, or something that is meant to appear like something that is real even though it isn't.  This brings up the debate as to the purpose of art itself.  If art is just an imitation then why do we need art and not the real thing?  What is the significance of even having art if it is just an imitation of something that we can have in real life like a chair?  Whats the point of having a painting of a chair instead of the actual chair?

17th century art vs 20th century art

I was reading the introduction and noticed that there was a comparison between 17th century style art and 20th century art.  While comparing the two of them one of the things that i started to wonder is can art still be considered art if its only a line on a paper? How can you define what is still art when it is just a line or even a dot like in some of today's art?