In the grail war, seven masters are to be chosen to summon avatars in a contest to obtain the holy grail. Three command spells imprint on the a master, which appear similar to tattoos. This allows the master to issue up to three commands which the servant must obey, even if against their will. If all three commands are used, the contract is dissolved. It's not uncommon for servants to consume the souls of their master under such circumstances, in order to sustain them until they can find a new master.
The master is typically the target, as it's the easiest way to break the pair. If a master loses their servant, they can take refuge in the church. A master with no servant isn't a threat, but they can form a contract with other masterless servants, or interfere with the grail war in other ways. As such the master is usually eliminated even if their servant is killed first.
Masters are normally chosen by the grail, but any person can form a contract with a servant. Normal people don't have the magical circut needed to sustain a servant, so if a contract is formed with a normal person the energy needed to sustain them has to be found elsewhere.