Marking is useful because it provides a rational mechanism for tough characters to protect weaker characters.
Before, if the king had two bodyguards they were bodyguards because 'they said they were bodyguards.' With no real mechanism to protect the king, any sufficiently prepared party could speed kill the king in a round or two, and then basically run away.
With marks, they can't easily move away from the bodyguards, and can't easily attack the king - the bodyguards distract them, disrupting their blows, and generally hurt them badly if they try to beat on the king. Suddenly they better respect the bodyguards.
Similarly the fighter can bodyguard the wizard for the party. How many times in fantasy have you read "keep the orks off me while I cast this spell" or some variation? With marks you have a mechanic to actually do that, rather than the DM occasionally being nice to you.
Before, if the king had two bodyguards they were bodyguards because 'they said they were bodyguards.' With no real mechanism to protect the king, any sufficiently prepared party could speed kill the king in a round or two, and then basically run away.
With marks, they can't easily move away from the bodyguards, and can't easily attack the king - the bodyguards distract them, disrupting their blows, and generally hurt them badly if they try to beat on the king. Suddenly they better respect the bodyguards.
Similarly the fighter can bodyguard the wizard for the party. How many times in fantasy have you read "keep the orks off me while I cast this spell" or some variation? With marks you have a mechanic to actually do that, rather than the DM occasionally being nice to you.