One of the more popular ideas is that blood is energy rich, and shedding blood releases that energy quickly and strongly.
It's something you have to be careful with though, since energy for magical purposes is better controlled and used within the body. Kind of like rationing too much water and food in a survival situation: the food is useless until it is inside you.
Using blood does certainly add a measure of sacrificial personality to a ritual. Other bodily fluids can work well, too, as long as the symbolism is right in the practitioner's mind.