The medical school wants to know that you are familiar with medicine as a career and with the clinical environment(s) that doctors work in. They also want to be sure that you have the time management skills and the fund of knowledge acquired from the pre-reqs to be able to learn what is taught to medical students.
The PhD program wants to be sure that you have a knowledge of medicine as a career and still want to do PhD in addition. Beyond that, PhD programs want applicants who will be a good fit with their labs... that you have an interest in specific types of work being done at that school or, at the very least, that you have specific technical skills that are in demand in at least some of the labs that have openings.
So, having bench skills, having good hands, having the ability to work with others and to learn new skills quickly are the sorts of things that will make you a more "in demand" PhD applicant and thus a more highly regarded MD/PhD over someone who has less of those experiences and attributes.