I have a US military issued backpack i got from a surplus store that I travel with.
1.) Because 99% of my trips it can fit everything I need in it.
2.) When I would travel to Guatemala, there was a practice in villages where people to sneak up behind travelers to cut the bottom of your pack, to try and steal your shit. So, I thought it would be best if I got something more heavy duty than a Jansport.
That being said, I always feel a little guilty because everyone thinks I'm in the service when I travel anywhere. I never pretend that I am, but it can get a little awkward at times.