“Why” is a really big topic for game design, but the simplest answer is that we want to abstract elements of reality and gamify them. How people represent elements of reality in a game are completely dependent on their interpretation or desired representation of the abstraction. That’s a lot of talk, so let’s use an example: combat. You can design combat an unlimited amount of ways around any core abstraction of combat you like such as endurance, strength of attack, timing, or even building combinations. When designing, to reach the most people, we need to use simple ideas that are easily accessible so the most people can understand the abstraction as easily as possible. In ICRPG and many d20 systems the core is accuracy of attack: roll a d20 plus any modifiers to see if you hit. That’s easily understood and reliably relatable.
But, I’ve been practicing civilian combatives for years, and even have run organizations and clubs, so maybe my abstraction of combat might include nuance such as defensive weapon concepts like “pass/jam/check” or timing related elements such as “baiting/defanging the snake” etc. Those concepts aren’t readily explained or understood, so they need more nuance of rules to express for others to understand them and quantify them, so they would only be fun for people who really, REALLY like crunchy combat systems. Those don’t sound fun to me at all, personally, haha, but those are possible abstractions based on my combat training.
As far as HP goes, I love HP. When represented as Vitality it represents my character’s ability to fight on. When the HP reaches 0 I can’t go on anymore. Games like D&D try to say that’s what HP represents, but then why are we tying Constitution to it, which represents a character’s physical fortitude? Why not Wisdom for willpower to fight through? HP is simply the abstraction of your character’s ability to fight on.
Also, as someone who is currently playing a system that uses Wounds and Conditions to represent a player’s “vitality” instead of HP: I friggin hate it (Savage Worlds, if you’re wondering). When unwounded, my players are fighting at their best and take chances, play bravely, and do cool stuff! But as soon as someone takes damage they are immediately penalized in some way or another for being wounded, normally in a negative penalty to their rolls, speed, or something else. They enter a state in which they’re afraid to enter the fray because of their penalties. The wound system has effectively created a Poor-Get-Poorer negative feedback loop. Wounds are an abstraction of your physical condition, which might be more realistic, but you effectively can only play at your best if you’re uninjured.
I’ll give an example of how that hurt my last game session. Mind you, they wanted to play a system that had more rules that would quantify more of their progression and ability. That alone has hurt our creative ability at the table. But I digress (I’ll be going back to ICRPG asap).
One of my players got shot in the gut, real bad. Half the party got captured and taken to a makeshift prison with no supplies and no way to heal up within the ruleset available to them. They got into a stealth/combat encounter throughout the floor, and the character who got shot was rolling a -3 to all of his rolls from his wound level (target is 4 for the system), speed reduced to a single space/turn, and his strength was reduced a die type from his level of wounds. He spent the night not being able to take any actions because he was afraid he would bleed out from exerting himself. He had a great time roleplaying, but that’s independent from the mechanics. Seriously, he was in a bloody heap curled up in a corner to avoid being seen.
Compare that to a character who only had 1 or 2 HP left: he still could have gotten up and skulked around, risking his livelihood, all while still being so fragile he could die in a hit. Instead, the mechanics of the wound system in place made him afraid to do anything rather than make big, risky plays to turn their situation around.
When designers think about mechanics we really have to look at the target audience and think about what experience we want to give them. A designer should know a player experiences a game in this order: aesthetics, dynamics, mechanics. So, designers start with making mechanics which ultimately changes the aesthetics (player experience). If you want to reach a specific audience, your mechanical iterations should move towards dynamics that will create pleasing aesthetics for them. That is exactly why there are so many different RPGs out there and why they’re needed.
Geez, I could nerd out on game design and continually be right or wrong and still have a great time.