Here is my take on why I think it's important to keep a personal diary in addition to having a blog.
Keeping a personal diary during our travels was a godsend and helped us to record our experiences during times when we couldn't access the Internet or had no access to a facility that could provide us with a computer. When we did visit Internet cafes, I could never fully write as I was always watching the time and wondering about the money being spent, plus it was hard to concentrate with kids playing shoot em up games nearby.
Since we were traveling light, we did not pack a laptop and instead had only a few electronic devices for basic necessities like an iPod for checking emails and of course a camera. The diary also allowed both of us to share a lot more personal details that we felt perhaps couldn't be shared with a wider audience and gave us something to do on long bus rides. It was also a great place to record email addresses of people encountered and especially in China, we drew lots of attention when we would pull out diaries and the people would gather around whispering in Chinese and looking at our writings or trying to decipher my handwriting that must've looked like an altogether foreign language.
Many would look at our diaries in awe and wonder what we were writing about and the few that understood a little of what we had said, would wonder even further about how much we had traveled and the diaries real purpose. My Moleskin diary was easily filled in no time and had to buy a much cheaper version in Central Asia, it was more for drawing graphs as it had boxes instead of lines on the pages. I admit, there were days where I did not feel like writing in it, I came up with excuses to avoid the work of catching up on the day's event, some days I had to catch up on an entire week. I am however, glad for having kept up with it. Also, recording our thoughts on paper allowed us to make detailed blog entries later, which otherwise may have forgotten.
It was also nice to flip through the pages later and look back on the places we had come from and imagine the people we had met and their lives being played out.