The key question is "When does the encounter end?"
Hit Points are an easy way to determine when an encounter ends, but is also flexible enough to create encounters of vastly different lengths and strategies. For example, if minions have few hitpoints, they may be killed quickly before the boss. If they have many hitpoints, it may be better to control them instead, and kill the boss first.
They also allow an easy mechanism for the healer role.
As well, hit points allow characters to increase in power without changing the basic fights. For example, a character increases in level and does +5 damage per round. But she also gains +50 hp. The two increases balance each other out. But the character is still more powerful than before.
It's possible to make other systems, but they are rarely as simple, flexible, and intuitive as hit points are. That's why hit points have survived for so long.
Hit Points are an easy way to determine when an encounter ends, but is also flexible enough to create encounters of vastly different lengths and strategies. For example, if minions have few hitpoints, they may be killed quickly before the boss. If they have many hitpoints, it may be better to control them instead, and kill the boss first.
They also allow an easy mechanism for the healer role.
As well, hit points allow characters to increase in power without changing the basic fights. For example, a character increases in level and does +5 damage per round. But she also gains +50 hp. The two increases balance each other out. But the character is still more powerful than before.
It's possible to make other systems, but they are rarely as simple, flexible, and intuitive as hit points are. That's why hit points have survived for so long.