Why B?

I'm not an instructor and I'm not completely sure that this is the right way to go about the question - but I let process of elimination guide me in this one: C, D, and E are all weaker statements--weak enough to be supported by the stronger language in the stimulus, so I ruled those out pretty quickly. A vs. B is what threw me off, but A (though very strong) is actually supported by the first part of the stimulus "Only poetry cannot be translated well" which means that not translated well --> poetry not poetry --> translated well or in other words all nonpoetic literature --> translated well. So this only left me with (B) where, as you said, "purpose" is not explicitly mentioned, so it seemed like the best answer choice here (although not immediately clear).

Process of elimination is indeed a great way to get here, but I think we have a stronger option in B. As you noted, the passage says nothing about purpose. We have no clue about the actual intent of poets!