1. You'll have more time to study for the MCAT if you haven't taken it yet. MD applicants: to be as early as possible in this application cycle (which, as we already know, gives you a better shot at getting into schools), you should be focusing on taking the MCAT no later than May 8 if you want your scores before you submit your AMCAS, or May/June if you are comfortable submitting AMCAS before you have scores. If you applied for '16, you could spend all summer studying MCAT, and still have time to retake it if necessary before applying.
2. You can use next year to solidify your GPA. Your academic metrics almost always improve in your senior year if you're a junior because you have more control over the courses you take and you're more acclimated to the college environment. If you're a senior whose GPA is not competitive at schools of interest, advisers can discuss various options to improve your GPA before applying.
3. You might secure stronger letters of recommendation. Again, if you're a junior, your classes are likely to get smaller next year, and you'll have more opportunity to forge relationships with your faculty, especially your thesis adviser. You'll also have this summer to work on garnering a letter from a supervisor or volunteer coordinator in a summer activity. If you're a senior, going on to post-bac class work could give you more chances to get to know faculty, or if you go on to work, you could have an excellent professional reference.
4. You can get your finances in order. Health professions school is expensive, as is the process of applying. Taking time away from school means that you may have to start repaying any student loans, but working full-time should allow you to make payments on loans (to defray some debt load) while also saving some money to put toward applications and future expenses. If you have poor credit, rebuilding your credit record may also pay off when taking out professional school loans.
5. You'll have more time to focus on the preparations required to apply. You have essays to write, letters of recommendations to gather, MCAT to study for, schools to research, as well as the rest of real life and figuring out what to do this summer. If you can't spend the time you need on application prep now (and secondary essay writing this summer), it might be better to start getting organized this year, but focus on applying next year.
6. "Everyone else is doing it." Only about 1/3 of Princeton applicants currently planning to apply for '14 are class of 2014 - the rest are taking time off before matriculating. Every student with whom we have talked about taking time off has benefited from it. They have all found something productive to do in their "glide years," and may be more attractive to admissions committees because of this new experience as well as the maturity gained from being in the 'real world' for awhile.
7. You can gain more experience and practice articulating your career interests, on paper and aloud. You can participate in activities that allow you to serve the community and to build the skills you need to be a health professional, so you can convince schools that you have a realistic understanding of what you're about to undertake. The more time you spend in these settings, the better you'll be when interviews come, and the easier it will be to focus on applications, since you'll have a more solid goal to work toward. If you need more experiences to back up your "gut feeling" that you "must" be a doctor or dentist or vet, by take the time to find those experiences. If you're having trouble writing your essay, or practicing interview answers, you may just need more time and experiences.
8. It can be hard for a junior to compete favorably with alums and post bacs who have rich life experience, and with seniors who have their complete academic history...graduation honors, undergrad thesis presentations, etc. Admissions committees have acknowledged that younger applicants often "suffer by comparison" to the older, more experienced applicant (the average age of successful applicants to US MD programs at anticipated matriculation is 24).
9. Life is short! Once you get to medical school, it becomes more difficult to take time off - you're more likely to have financial concerns, family concerns, and a professional schedule that will keep you from, say, traveling to Africa for six months, or learning to skydive, or pursuing independent research, or going to culinary school.
10. Your brain could use the break. [Premed] academics are rigorous, perhaps even grueling at times. You may just want some time to take a break from academics after 18 years of school, so that you can return renewed to the rigor of health professions school course work. Health professions school (and the support your alma mater provides in working with you to get there) will still be there for you if you go and do these things and return to the application process later.