I guess the most basic explanation of his importance is that he made the absurd artistic, giving rise to the dada movement as the springboard for most visual art today -- including broadcast images.
I recently read a good book about the dada artists, I think it's called "Dada-Art and Anti-art." It absolutely fascinates me how much the dada and post-modern artists have influenced the visual and spoken medium since the early 1920's.
You're not the only one to be taken by Duchamp's sculpture. I think about it often, and what it represents from the period. It was so simple, yet innovative, for Duchamp to display a commercially manufactured urinal as art. Most imagery today can be traced back to the dadaists -- one good modern example is the use of absurd metaphors and imagery in advertising.
Good night to the Session -- the sculpture:
A jelly containing a clock;
Where they say, ‘From the way that you gulped you're
Therapeutically thrilled by the shock!’
-- It's the Shock, of, alas, Recognition
At what's yearly presented as new
Since first seen at Duchamps' exhibition
‘Des Maudits’, in Nineteen-O-Two.-- Ted Pauker, A Grouchy Good Night to the Academic Year (with acknowledgements to W.M. Praed).