Blood is a specialized bodily fluid that is responsible for both delivering nutrients and oxygen to the cells as well as transporting metabolic waste products away from those cells. Immortals, vampires and hybrids need to drink blood to function. If a vampiric being cannot feed on blood for an extended period of time, they will weaken to the point of desiccation, which will essentially turn them into a statue-like state until someone else feeds them. Blood is also what provides vampiric beings with their supernatural abilities, such as enhanced strength, speed, agility, senses, healing, and mind control, and as a result, the strength of these abilities can be dependent on how recently they've fed; starvation can weaken a vampiric being's powers considerably, whereas someone who has just fed on a significant amount of blood will experience a slight increase in the strength of these powers. Most vampires drink human blood, but there are some who choose to exclusively drink animal blood, such as Stefan Salvatore, or vampire blood, such as Mikael. However, animal blood does not provide the same level of nourishment to vampires as human blood does, which is why most vampires won't even consider it unless they have no other option.
Witches can integrate the use of blood into their magical practice to strengthen certain types of spells, which range from simple and minor locator spells to more powerful and complex curses and hexes whose effects can be so devastating that they can even kill a human or supernatural creature.