People get hurt. It happens. Such is life. No one gets very far without pain. Scars make you who you are.
There are so many great (sorry) things about injuries in the written world. How did it happen? When did it happen? How is the recovery going physically? Emotionally?
To give you a quick example: I have this huge scar on my knee cap. I've had it for about 8 years now. How did I get it? Well... One day (in high school) me and a few friends were goofing off in the parking lot. I decided to climb onto the running board of my girl friend's bronco while she was driving around. She was moving slow, but she turned and I couldn't hold on. So, I flew off and hit the ground. My head bounced. The blacktop ripped away the knee in my jeans along with the skin, nearly down to my bone. She freaked out because she thought she ran me over. It was interesting, and for the rest of my life, when someone sees that scar, that is the story I get to tell them.
Everyone has a scar like that. When we get drunk we like to compare our battle trophies. Its how we behave. In some sense, its a bit of a my-horse-is-bigger-than-your-horse thing. Regardless, why shouldn't our characters do it too?
Injuries and scars run deep emotionally, too. How does a character recover from things like hurt feelings, wounded pride, or a broken heart? These are all things you should answer when you're working on your character development.
Of course, injuries can be used in writing as comedic relief, too. Slapstick comedy has worked forever! How long have the Three Stooges been popular?
A big thing to think about when your character gets injured is how others are going to react to the injury. Some people (like me) don't deal well with blood. Some think the gore is awesome. Sometimes, when someone gets hurt, their significant other can't handle the burden of having to take care of them. As well as the physical side of this, remember that emotional baggage can have a huge impact. Past injuries can temper our future actions.