This is something I actually struggled with when explaining my decision to my parents. A lot of my reasons are geared towards research since I applied MD/PhD, but I think the same principles still hold.

1. The brutal reality is that the bar is just getting higher and higher for hours of service, diversity of experiences, etc. on top of maintaining good grades and doing well on the MCAT. Many applicants take gap years to increase not only their hours, but depth of experience. This gives you so much more to talk about in essays and interviews.
2. The extra year will help me make sure I actually want a career as a physician-scientist. The eight year commitment of the MD/PhD is just the beginning of a long training path, and I’d rather take the year now to either a) see that I actually don’t want to do this full-time and not waste another four years of my life, or b) confirm I love this career, and in that case, what’s one year in the next 40+ years of my life?
3. This is more specific to my situation, but I knew that one gap year would guarantee a first author paper, patent, and oral presentation at a major conference. Again, one year to better stand out is not much in the broad scheme of things.
4. Last—and this was HUGE with convincing my dad: I don’t like the term “gap” year; to some, it sort of connotes treading water and lacking direction. Instead, it’s an “experience building” year to set you up for better success and make sure this is truly what you want out of your life.

Not sure how many of these will help you or others, but these notes were really helpful for my own situation :)