Whats the point of building armor?

I just played rammus in an ARAM game and with W active i have over 400 armor, yet the tristana on the enemy team (who had no armor pen items) cut through me like i was made of butter. If thats the case what is the point of actually building armor items since the literally have no effect. Oh and i should also mention i was at 4k+ health so its not like i was squishy. here is a link to a screenshot of what i was dealing with close to the end of the game. http://imgur.com/a/EbKYl This seems pretty broken to me.
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