To answer the 'how to split' question first - I use a heavy knife and cut down between the knuckles, that seems to be the toughest part - the the foot splits to the heel rather easily. And as I said originally, and others repeated - don't cut yourself.
As to why - the fur on the bottom of the foot is about twice as long as you think it is when viewed from 'outside'. And when cutting the hair on a non-split foot you have to dig down with the scissors (which are on somewhat of an angle). This wastes hair and doesn't allow you to get a chunk as long or as clean as you might like. Once the foot is split you cut straight in for the hair you want. You have a better view of the quality of hair, and you get a cleaner snip.
After working with it so often that makes sense. I just never thought of splitting the foot. I will now.