Blood is a symbol of life, our life source in fact. It is the most intimate part of us that we can remove from our bodies and use as an empowerment to our magick. It is used in inks to connect ourself more to whatever we mark. It is used to "feed" crystals (animal blood). It is also used to offer personal energy to spirits. It is the ultimate "sacrifice" to shed our blood for a spirit or for our craft in general. Blood is very sacred.
Generally anything that either needs life or personal energy "marked" or "offered" has a use for blood. However, in no way is it necessary. Many are too squeamish to use it! It only takes one single drop, because it is symbolic. But witches have also used menstral blood, urine, hair, saliva, etc as a substitute for personal energy (not offerings). And red wine as a substitute in offerings. Anyone who has been to a Christian mass can see the link between blood and red wine still continues and is quite popular.
Animal blood, in modern times, unless you hunt (I know quite a few deer/pheasant hunters) you can go to your local butcher. "Duck blood soup" among other cultural recipes require blood as ingredients, and butchers usually have a variety on hand. So don't feel odd asking them. No explaination required. Just look up a recipe to use as an alibi! Lol