Breathing exercises help to induce a natural state of calm which is an effective precursor to entering a trance state. In this state, raising, directing and programming energy is much easier.
You'll also find it easier to do things like void meditation, or really any task where you need to still your brain and focus your thoughts on a singular thing, as a relaxed mind is less inclined to be running a mile a minute.
What you can focus on can vary from breathing exercise to breathing exercise; you may be focused on the breath itself, like the counting you do for each breath, to help get a feel for it. I like trying to focus on the feel of energy as I breathe it in, feeling it in my aura and in my chakras, as this is much easier to do in a relaxed state.
Just remember that it's always to be relaxed breathing and never pushed. I get shortness of breath at times so I tend to hold the breath in a bit longer than my exhale to avoid the feeling of pushing myself and to really allow my body the opportunity to get that oxygen. Otherwise I find myself breathing in faster on the inhale in an attempt to catch my breath and that's inadvisable.