Flight out of Busan

Hi Everyone! I am planning an upcoming trip and been observing this sub for a while. I always see people doing Seoul. Then going to Busan. Finally coming back to Seoul to take a flight back home and end the trip. Was wondering if there is a specific reason people go back to Seoul and not take a flight out of Busan directly? Is it because of costs? Flight frequency? Lack of connectivity? These are the only obvious reasons I could think of.

Be cause, I found a good flight out of Busan. Similar duration and not too much difference in costs? Any other factors I should consider regarding flying out of Busan?