on Writers Helping Writers:

In a story, when we put characters in dangerous or unfamiliar situations, they can get hurt, and when they do, it makes everything harder. They might have reduced mobility, the pain may make it hard to think, or the injury makes something they must do (win a fight, escape a threat, or keep their independence, for example) all but impossible.

Injuries and pain lead to problems, meaning they can be great for a story as they generate tension and conflict. Too, readers become more invested when something happens to a character, either because they care about the character and want them to be okay, or the incident leads to a rush of schadenfreude because a nasty character is getting what they deserve.

A writer’s mindset is all about How can I make things more difficult for this character? meaning it can be tempting to pile on the injuries and pain, but this can lead to other story problems. Too many ouchies and a character’s not going to be able to continue their quest, and that means not achieving their goal (or worse, causing the author to manufacture a ‘Hail Mary rescue’ that feels contrived). Or, if injuries are piling up like a serial killer’s body count, readers might get angry and feel the author is going too far. So we really want to find the sweet spot of making things hard, not impossible for the character.