Just like how Neo from the Matrix learns via computer software, REM sleep is a virtual realm where you learn, practice, and memorize language and motor skills. The epiphenomena of this learning process, aka dreaming, may be recalled at times, but seems unimportant since you're not suppose to really be aware of them in the first place. It's kinda like waking up in your dream when what you're really suppose to be doing is sleeping in your dream.
And since REM is for these procedural kinds of memory it would only seem natural that NREM is for the consolidation of declarative memory. These studies are still in the works and I am anticipating exciting findings very soon.