Ah yes, great point. The critical thinking, introspection, and analysis that goes into writing definitely strengthens neural pathways.
You just reminded me of another one: teaching.
One study showed that older siblings, on average, tend to be slightly smarter than their younger siblings. One plausible rationale may be because they spend time teaching their younger brothers & sisters.
http://www.bioedonline.org/news/news.cfm?art=3401
If you've ever taught someone a subject, and found yourself walking away knowing that subject better than you did before, you're experiencing a similar phenomenon.
The act of teaching a subject requires you to understand it well enough to be able to synthesize, then explain it to someone else.