Typically, these holders of MD/PhD degrees spend a significant amount of time in the research setting each week (writing grants, conducting research, mentoring and teaching other members of the lab team, writing journal articles, lecturing and giving presentations to peers at professional meetings) as well as providing some clinical care or supervising residents and fellows who provide care or serving as a consultant to primary care providers and specialists.
The training is significantly different. For the MD/PhD one does some lab rotations during the summer after first year (sometimes the summer before as well), then takes several years of graduate school classes & does reserach including writing and defending original research (the doctoral dissertation) before doing the clinical rotations that make up the 3rd & 4th years of medical school.
If you have done a significant amount of research before applying to med school, love research, have a very supportive mentor/PI who will write a very supportive LOR, and you have had the exposure to clinical care that leads you to want to spend some time in patient care (otherwise, you might as well go to a PhD program & skip MD), then the MD/PhD may be a good fit for you.