My understanding is that if you have meaningful work experience before medical school (i.e. not flipping burgers or cashiering at the local corner store) then you can put it down on there. Many people who entered medical school straight out of college don't have work experience at all, so this would be something that sets you apart. And clearly most medical students don't have meaningful work experience while in medical school because they don't have enough time. I had a handful of jobs with lots of responsibility prior to medical school (i.e. coordinator for a depeartment of a large newspaper, etc), so I'll be putting that stuff on there for sure.
I'm including research projects and a work experience (since I've only ever had the one) from before med school, but leaving out things like volunteer work and awards, that will be less important for my chosen specialty.