As I understand it, you start cannibalizing muscle tissue for energy at a certain point... so adding protein prevents that from happening, supposedly.
I just find the idea of a consistent intake of a relatively complete energy source much easier to deal with mentally as opposed to fiddling away with the numbers in my head all the time. If I know I need to consume "X" volume in "Y" hours and I will then be getting as much caloric input as I can absorb, then I won't worry about under or overeating. Also, I'm planning on doing all the training miles solo, without wanting to bring a lock and stop at convenience stores along the way.