But, an 8 year program does give you more options. That additional year of undergraduate college means you have more time to do research; more time to engage in volunteering; more time to arrange a study abroad program; more time to study for the MCAT if that is required by the program.
Are these things necessary? No. But there are things that many students might like to be involved in during college.
The 6 year, 7 year and 8 year BS/MD programs all have their advantages and disadvantages, but the longer the program the more options you will have. This is why I can never answer the question of what length program is the best. It depends on what is best for each student. And the answer to that, depends on you.