Specificity is always good. It allows the interviewer to imagine you in the scenario, imagine you performing a task, and hopefully succeeding at that task. Like, if your volunteer work is teaching kids with learning disabilities, give me a brief overview of how long you've been doing that work and the name of the organization, tell me how you get kids with different disabilities to understand difficult concepts, tell me a story about one kid in particular who you tutored and how you helped him study for a difficult test and even though he's dyslexic, in the end, he was able to get an A on his test, then tell me how your ability to communicate difficult concepts is going to help you communicate with your patients when you're a doctor— you're going to have patients of all different educational backgrounds, ages, etc. and you need all of them to understand their conditions and their treatments. Also, teaching is important for doctors because as a doctor you're going to have to teach medical students and residents, and your experience as a volunteer tutor has really helped you improve your teaching skills.
Again, the more specific your story is, the more cute details you include about this kid, the more passionate you're going to seem about your work. It also makes for a better story and it's more fun and engaging for your interviewer.