I am sure this happens more frequently than you can imagine--I am Not Your Mama so no judgement here, just examples. Let's say you want to be a Marine and your appointment is to USAFA? Let's say you want to major in Philosophy or Political Science and do not want to take those engineering and math classes for your Army commission? Maybe your girlfriend is going to the state college and you want to go together? Maybe you just don't want to live under the UCMJ and under lock and key for four years or you just want to live closer to your family or maybe your ROTC school is Stanford or MIT. There are many pathways to earning a commission and service to the Nation. It is a big decision.
A million years ago when I was a HS senior, I applied for the the SAs and for ROTC scholarships. My father was active duty AF at the time, so I knew about military life. But after we visited the AFA, I said "NO WAY." I had my nomination but had not heard yet about an appointment or not. My parents said I should wait and turn it down after I received an appointment if I didn't want it. I knew that there was no chance I would be "able" to turn it down it was offered to me... so I pulled my application before I heard results. I ultimately ended up with a 3 year AF and a 4 year Army scholarship (and accepted the Army). I knew that academy life was not what I wanted. Everything worked out very well, and I know I made the right decision FOR ME. It is a difficult decision... talk to parents, guidance counselors, mentors, friends, academy and ROTC cadets and alumni, anyone whose insight you trust and then decide FOR YOU. Good luck.