Sacrifice can be made because a witch needs the dead animal (in which case I’d question why they don’t just find an animal that’s already dead) or it can be incorporated into a necessity killing, for example witches who hunt for food. This is often seen as more morally justifiable, particularly if the whole of the animal is put to use and not wasted.
The key idea behind animal sacrifice is to raise energy and power. Some witches believe that the life force of the animal is channelled into their magic. Others align the act of killing to symbolise the power of the witch. There are in my opinion significantly more effective ways to raise energy and I actually see the whole animal sacrifice thing as posturing and a bit clichéd these days.
Image http://www.sxc.hu/browse.phtml?f=view&id=1008596 (bugdog)