I'll first preface with saying that I did not march DCI, but only because I wasn't familiar with drum corps until I was 23. However, I have and still do march DCA and do not see myself stopping anytime soon.
A) I was first drawn to drum corps because as a recent college graduate looking for a job as a music educator I wanted to get experience in marching. I did not attend a high school or college with any type of marching program. To make myself more marketable for band teaching positions, I joined a DCA corps to gain experience for my resume.
B) I was compelled to stay in it because I fell in love with it. I love having a way to still learn and grow as a musician all the while fostering relationships with the people. It also is a great way for me to stay fit during the summer months off when I'd be tempted to be lazy. Knowing I'll have a drum attached to me all day every weekend keeps me in the gym during the week!
C)It got me a good job in a tough job market for band directors.