Footing the nock end helps protect the nock end when the nocks are hit by other arrows while shooting.
To help with pulling the footed carbons from foam targets, before you install the aluminum tubes on the arrow, taper the end of the tube. Helps quite a bit.
I don't use carbons with the HIT inserts (yet), but have been told that is you install the aluminum footing so that it bears on the point, it supports the point and makes the end much stronger. Oh, yeah, you have to glue the footing onto the arrow, in that case, can't use a wedge fit.