When your toes are pointed ahead, your lower leg is able to be in a more correct position than if your toes were pointed outwards. When your lower legs are correct, you have a much less risk of confusing the horse with accidental aids, or provoking a sensitive horse.
What my trainers have me do to keep my toes from pointing outwards, is to feel like I'm putting my weight into my pinky toe. I position my foot as if I were standing on the outer edge of my foot. (I hope this makes sense.) When it feels werid, then you're doing it right! Practice this weirdness until it feels normal and voila! Fixed toe position. My trainers have been yelling at me "Toes ahead!" for a year now and I am seeing huge improvements when I compare photos of my riding.
If you need more clarification, let me know.