Can you imagine if every time someone talked about healing, they called it "adding blood" instead? In China, the word people use for "health" is "xue," which means "blood" (and is pronounced a bit like "shweh"). Traditionally in Chinese role-playing games, the health bar (or "blood bar") is red, instead of green.
Now when you think about it, having a "blood bar" does make a certain sort of sense. After all, when you get hit by monsters, you lose blood, and any healing you take from others would have to somehow restore your blood to your body as well as sealing up all the holes in your flesh. Of course without healing, all those holes in the flesh would also prevent a warrior from swinging his sword around so freely, or at least make him limp a bit. But realism isn't really the issue here -- the idea of "blood" or "health" as a measurable quantity is just something we need as a symbol to represent the video game mechanics in an emotionally meaningful way.
A game like WoW can't possibly be as complicated as real life; it would hardly be as fun as it is if it were. Instead, it needs to use real life metaphors as an access point to get you involved in the game, while in the end it's still all about numbers. Stripped of metaphorical words like "health" (or "blood"), playing World of Warcraft might look a bit like this:
Player 4837 says, "I'll reduce your unit's primary points with my unit's special 'large-scale point reduction ability!' Pwned you!! haha!" only to be countered with Player 7490's response: "Oho! but my unit can use my secondary points to exchange for primary points, and make up for this loss! Noob!"Talk about boring! But underneath all the "fireballs" and "greater heals," this shifting of numbers around is exactly what we're doing when we play, no matter where we are or what language we speak.
In China, of course, the points and numbers are exactly the same, but it makes sense that the underlying metaphor would be somewhat different. For them, "adding blood" to a wounded teammate feels just as natural as when we say we are "healing" them -- but when you translate their "blood" metaphor into our language, it gets pretty weird!