In an email yesterday I remarked that morning pages were like Stoic meditation but with a pen. Stoic meditation acquires its subject as part of the process, when a thought arises it is examined with deliberate, objective curiosity.
See this post from a couple of years ago for more thoughts on types of meditation.
Unlike meditating on a koan, with Stoic meditation you don't know what you'll be thinking about in advance; it's the same with morning pages - you let the mind wander and write down whatever emerges.
As things progress I regard these morning sessions more and more as meditations rather than just writing - there is definitely a quality to them that transcends just putting words on the page. The more I treat them as such, and approach them is this way, the more I feel I will get from them.