The beauty of this section is that it allows the interviewers/programs to get a feel for your background and see what you have done previously. I can feasibly see only two reasons to not list anything in this section: 1 - you haven't had a job...ever. If that's the case, leave it blank, then promptly get a paper route or start a lemonade stand and 2 - your employment was illegal and you prefer to not incriminate yourself on your residency application. In other words, don't leave the present and past employment section blank.
So what should you include? Again, everyone does this a bit differently, and, what is nice about this section is that you can decide which jobs you list and you can even decide which jobs not to list for whatever reason. In my application I definitely listed being a TA in undergrad, being a research assistant, and other healthcare-related jobs. However, I also listed a prior job at a music publishing company because I felt it further validated my interest in music as well as working at my college bookstore one summer as it demonstrated my interest in reading/learning etc. More often than not, your prior employment is a reflection of your interests and will further substantiate your interests/hobbies. Where this isn't the case, listing a job that was less-than-glamorous will demonstrate that you are a hard worker and simply needed to pay rent/buy books and didn't care that the job was in whatever-horrible-job-you-had[\b] for that summer in college.