i concur with Pat, they keep the mass down, the spine up, and the foc closer to zero.
the fOC being closer to zero ha some huge advantages in greater distances.
when shooting higher foc arrows- and yes, we know they work for penetration, bone busting and paradox recovery, but to me their biggest disadvantage is after some time in the air, as the velocity starts to peel off, the heavier forend, will start to tip the arrow forward/downward, and this changes the flight attitude. so the arrow starts to aim more and more downwards until its sticks into the earth.
i have heard many folk who shoot EFOC arrows, say that they seem to drop like stones after 30 yards.
if you took two arrows exactly the same weight and fletches etc- one EFOC one normal FOC- and look at their distances shot out of the same bow.
Now, the breasted arrow does the opposite, with the FOC, being closer to zero %, as the velocity peels away, the arrow will maintain a more level flight attitude- assuming a "glide path" and probably landing with a flatter attitude. In turn giving more range.
in flight shooting, the quicker the arrow recovers from paradox is also important,as any energy wasted with less than perfect flight , detracts from distance flown.
would love to see someone do a test with all 3 different types of arrows, same material,shot from a shooting machine.