There's a few theories:
1. Unrelated interest/hobby: Undergrad is (or use to be prior to the internet) a chance in a lifetime to study something in depth that you are interested in. So, nourish your soul and develop that which you are passionate about. Just remember to fit in the medical school and MCAT prereqs. (FYI - A group of theater majors will be socially somewhat different than a group of science majors, business majors, etc. I think some ADCOMs like having a diverse class of not all biology students or not all science majors. This provides for a diverse group of physicians later on.)
2. $$$$$$$: Undergrad should be used to acquire profitable skills. The sooner you can start saving money in interest bearing accounts (over the summers, etc.), the more money will compound throughout your life, and you should have a nice emergency fund. (It's been said that putting $4000 in a 14 year old's roth ira is projected to become $1 million tax free by retirement age. That's the power of interest.)
3. Complimentary skills: Undergrad can be used to acquire skills that will COMPLIMENT your medical career. Some people want to do more than treat patients or have hybrid careers. Research is probably the most popular example of this. Some, but not all, research jobs prefer or require a biology major or BS. And some students will pick an undergraduate major that has to do with the medical SPECIALTY they think they'll want to apply for.
4. Back up plan: Undergrad can be used to train for a back-up plan. Should you decide you are no longer interested in medicine or if you don't get into medical school, maybe there's a second choice undergrad can prepare you for.
5. Med school prep: Some believe that undergrad be used to get a jump-start on medical school by picking a major that will include classes with the same name as classes taught in medical school: Immunology, hematology, etc. (Double check this "fact," as I've heard that medical school classes are very different from undergrad requiring students to "unlearn" stuff. Does this #5 really work? Don't ask me. It's a good question.)
Feel free to add any theories I've left out.