Well, you would never say, “Here is a poetry I wrote”, but you would say, “Here is some poetry I wrote”. The poetry you’re talking about there is simply a sample of the larger thing: all poetry. And if you say, “Here is poetry I wrote” you could be talking about one poem, or several poems, but the ‘some’ is implied. You’re talking about it as a sample of something bigger. It’s like ‘water’. If you said, “There’s water on the floor”, you would mean “There’s some water on the floor”, some of all the water there is. Poetry is like water, and a poem is like a puddle.